diff --git a/.rubocop.common.yml b/.rubocop.common.yml
index d896ed7ffa4..57bb3122f84 100644
--- a/.rubocop.common.yml
+++ b/.rubocop.common.yml
@@ -96,6 +96,8 @@ Performance/StringInclude:
   Severity: error
 Performance/StringReplacement:
   Severity: error
+Performance/Sum:
+  Severity: error
 Performance/TimesMap:
   Severity: error
 
diff --git a/app/models/conditional_release/stats.rb b/app/models/conditional_release/stats.rb
index b152890efb7..840a6ff7237 100644
--- a/app/models/conditional_release/stats.rb
+++ b/app/models/conditional_release/stats.rb
@@ -140,7 +140,7 @@ module ConditionalRelease
         new_scores = Array.wrap(new_score_or_scores).compact
         return unless old_score && new_scores.present?
 
-        average = new_scores.reduce(&:+) / new_scores.length
+        average = new_scores.sum / new_scores.length
         percentage_points_improvement = average - old_score
         return 1 if percentage_points_improvement >= 0.03
         return -1 if percentage_points_improvement <= -0.03
diff --git a/app/models/group_category.rb b/app/models/group_category.rb
index 5adccfaa668..968ad9edace 100644
--- a/app/models/group_category.rb
+++ b/app/models/group_category.rb
@@ -474,7 +474,7 @@ class GroupCategory < ActiveRecord::Base
                             counts = User.joins(:not_ended_enrollments)
                                          .where(enrollments: { course_id: context, type: 'StudentEnrollment' })
                                          .distinct('user_id').group('course_section_id').count
-                            @create_group_count = counts.values.map { |count| count / @create_group_member_count.to_f }.map(&:ceil).sum
+                            @create_group_count = counts.values.map { |count| count / @create_group_member_count.to_f }.sum(&:ceil)
                           else
                             (unassigned_users.to_a.length.to_f / @create_group_member_count).ceil
                           end
@@ -660,7 +660,7 @@ class GroupCategory < ActiveRecord::Base
         end
         @group_distributions[section_id] = dist
       end
-      if @group_distributions.values.map(&:count).sum != @groups.count || @group_distributions.any? { |k, v| v.sum != user_counts[k] }
+      if @group_distributions.values.sum(&:count) != @groups.count || @group_distributions.any? { |k, v| v.sum != user_counts[k] }
         raise "user/group count mismatch" # we should make sure this works before going any further
       end
 
diff --git a/app/models/media_object.rb b/app/models/media_object.rb
index 74adc996f9d..19bb166afef 100644
--- a/app/models/media_object.rb
+++ b/app/models/media_object.rb
@@ -227,7 +227,7 @@ class MediaObject < ActiveRecord::Base
         self.max_size = [self.max_size || 0, asset[:size].to_i].max
       end
     end
-    self.total_size = [self.max_size || 0, assets.map { |a| (a[:size] || 0).to_i }.sum].max
+    self.total_size = [self.max_size || 0, assets.sum { |a| (a[:size] || 0).to_i }].max
     self.save
     ensure_attachment
     self.data
diff --git a/app/models/quizzes/quiz_outcome_result_builder.rb b/app/models/quizzes/quiz_outcome_result_builder.rb
index e82b8ae1d9a..e2318da05d5 100644
--- a/app/models/quizzes/quiz_outcome_result_builder.rb
+++ b/app/models/quizzes/quiz_outcome_result_builder.rb
@@ -121,8 +121,7 @@ module Quizzes
         end
 
         result.possible = cached_questions_and_answers.map(&:first)
-                                                      .map { |h| h['points_possible'] }
-                                                      .inject(:+)
+                                                      .sum { |h| h['points_possible'] }
 
         # do not create a result if no points are possible.
         if result.possible == 0
@@ -132,8 +131,7 @@ module Quizzes
         end
 
         result.score = cached_questions_and_answers.map(&:last)
-                                                   .map { |h| h[:points] }
-                                                   .inject(:+)
+                                                   .sum { |h| h[:points] }
 
         result.calculate_percent!
         result.mastery = determine_mastery(result, alignment)
diff --git a/app/models/quizzes/quiz_regrader/submission.rb b/app/models/quizzes/quiz_regrader/submission.rb
index d5c4181a039..e0c6f9d7592 100644
--- a/app/models/quizzes/quiz_regrader/submission.rb
+++ b/app/models/quizzes/quiz_regrader/submission.rb
@@ -43,7 +43,7 @@ class Quizzes::QuizRegrader::Submission
 
   def rescored_submission
     previous_score = submission.score_before_regrade || submission.score
-    submission.score += answers_to_grade.map(&:regrade!).inject(&:+) || 0
+    submission.score += answers_to_grade.sum(&:regrade!)
     submission.score_before_regrade = previous_score
     submission.quiz_data = regraded_question_data
     submission
diff --git a/app/models/quizzes/quiz_statistics/item_analysis/summary.rb b/app/models/quizzes/quiz_statistics/item_analysis/summary.rb
index 8814369056b..032cd56ccb9 100644
--- a/app/models/quizzes/quiz_statistics/item_analysis/summary.rb
+++ b/app/models/quizzes/quiz_statistics/item_analysis/summary.rb
@@ -115,7 +115,7 @@ class Quizzes::QuizStatistics::ItemAnalysis::Summary
       size = items.size
 
       if size > 1 && variance != 0
-        variance_sum = items.map(&:variance).sum
+        variance_sum = items.sum(&:variance)
         size / (size - 1.0) * (1 - (variance_sum / variance))
       else
         nil
diff --git a/app/models/quizzes/quiz_statistics/student_analysis.rb b/app/models/quizzes/quiz_statistics/student_analysis.rb
index 4a1798b8dd0..4b399c853f3 100644
--- a/app/models/quizzes/quiz_statistics/student_analysis.rb
+++ b/app/models/quizzes/quiz_statistics/student_analysis.rb
@@ -420,7 +420,7 @@ class Quizzes::QuizStatistics::StudentAnalysis < Quizzes::QuizStatistics::Report
 
     question = Quizzes::QuizQuestion::Base.from_question_data(question).stats(responses)
     none = {
-      :responses => question[:responses] - question[:answers].map { |a| a[:responses] || 0 }.sum,
+      :responses => question[:responses] - question[:answers].sum { |a| a[:responses] || 0 },
       :id => "none",
       :weight => 0,
       :text => I18n.t('statistics.no_answer', "No Answer"),
diff --git a/app/models/rubric.rb b/app/models/rubric.rb
index c3ba1dfe77a..8e326e1d2c1 100644
--- a/app/models/rubric.rb
+++ b/app/models/rubric.rb
@@ -401,7 +401,7 @@ class Rubric < ActiveRecord::Base
   end
 
   def total_points_from_criteria(criteria)
-    criteria.reject { |c| c[:ignore_for_scoring] }.map { |c| c[:points] }.reduce(:+)
+    criteria.reject { |c| c[:ignore_for_scoring] }.sum { |c| c[:points] }
   end
 
   # undo innocuous changes introduced by migrations which break `will_change_with_update?`
diff --git a/gems/multipart/lib/multipart/sequenced_stream.rb b/gems/multipart/lib/multipart/sequenced_stream.rb
index 25d706116c3..6f2e6c6b895 100644
--- a/gems/multipart/lib/multipart/sequenced_stream.rb
+++ b/gems/multipart/lib/multipart/sequenced_stream.rb
@@ -24,7 +24,7 @@ module Multipart
     end
 
     def size
-      @streams.map(&:size).sum
+      @streams.sum(&:size)
     end
 
     def read(size = nil, outbuf = +"")
diff --git a/lib/score_statistics_generator.rb b/lib/score_statistics_generator.rb
index 993444760de..4d3c5ac88ef 100644
--- a/lib/score_statistics_generator.rb
+++ b/lib/score_statistics_generator.rb
@@ -136,7 +136,7 @@ class ScoreStatisticsGenerator
       return
     end
 
-    average = current_scores.map(&:to_d).sum / BigDecimal(score_count)
+    average = current_scores.sum(&:to_d) / BigDecimal(score_count)
 
     # This is a safeguard to avoid blowing up due to database storage which is set to be a decimal with a precision of 8
     # and a scale of 2. And really, what are you even doing awarding 1,000,000% or over in a course?
diff --git a/lib/simple_stats.rb b/lib/simple_stats.rb
index cf187190ac1..c826d424022 100644
--- a/lib/simple_stats.rb
+++ b/lib/simple_stats.rb
@@ -23,7 +23,7 @@ module SimpleStats
 
     divisor = type == :population ? items.length : items.length - 1
     mean = items.sum / items.length.to_f
-    sum = items.map { |item| (item - mean)**2 }.sum
+    sum = items.sum { |item| (item - mean)**2 }
     (sum / divisor).to_f
   end
   module_function :variance
diff --git a/lib/sis/csv/import_refactored.rb b/lib/sis/csv/import_refactored.rb
index fc1b14f56c3..a78e7f04154 100644
--- a/lib/sis/csv/import_refactored.rb
+++ b/lib/sis/csv/import_refactored.rb
@@ -211,7 +211,7 @@ module SIS
 
       def update_progress
         completed_count = @batch.parallel_importers.where(workflow_state: "completed").count
-        current_progress = [(completed_count.to_f * 100 / @parallel_importers.values.map(&:count).sum).round, 99].min
+        current_progress = [(completed_count.to_f * 100 / @parallel_importers.values.sum(&:count)).round, 99].min
         SisBatch.where(:id => @batch).where("progress IS NULL or progress < ?", current_progress).update_all(progress: current_progress)
       end
 
@@ -321,7 +321,7 @@ module SIS
           end
         end
         @parallel_importers.each do |type, importers|
-          @batch.data[:counts][type.to_s.pluralize.to_sym] = importers.map(&:rows_processed).sum
+          @batch.data[:counts][type.to_s.pluralize.to_sym] = importers.sum(&:rows_processed)
         end
         finish
       end
diff --git a/lib/tasks/canvas.rake b/lib/tasks/canvas.rake
index 25a503347d2..e22b6ac924a 100644
--- a/lib/tasks/canvas.rake
+++ b/lib/tasks/canvas.rake
@@ -81,7 +81,7 @@ unless $canvas_tasks_loaded
         end
       end
 
-      combined_time = batch_times.reduce(:+)
+      combined_time = batch_times.sum
 
       puts(
         "Finished compiling assets in #{real_time.round(2)}s. " +
diff --git a/spec/migrations/fix_points_possible_sums_in_quizzes_spec.rb b/spec/migrations/fix_points_possible_sums_in_quizzes_spec.rb
index f6a07084da1..c0fed6fda62 100644
--- a/spec/migrations/fix_points_possible_sums_in_quizzes_spec.rb
+++ b/spec/migrations/fix_points_possible_sums_in_quizzes_spec.rb
@@ -38,7 +38,8 @@ describe 'FixPointsPossibleSumsInQuizzes' do
 
   before do
     # creates rounding error just like the real thing!
-    points = Array.new(question_count) { question_points }.inject(:+) # .sum uses the native ruby 2.4 sum if the first element is numeric in rails 5.1 ...and doesn't quite give the same answer :/
+    # .sum uses the native ruby 2.4 sum if the first element is numeric in rails 5.1 ...and doesn't quite give the same answer :/
+    points = Array.new(question_count) { question_points }.inject(:+) # rubocop:disable Performance/Sum
     allow(Quizzes::Quiz).to receive(:count_points_possible).and_return(points)
   end
 
diff --git a/spec/models/group_category_spec.rb b/spec/models/group_category_spec.rb
index 80edaa72b3a..b1337e5bf9b 100644
--- a/spec/models/group_category_spec.rb
+++ b/spec/models/group_category_spec.rb
@@ -727,7 +727,7 @@ def assert_random_group_assignment(category, course, initial_spread, result_spre
   end
 
   # set up course users
-  user_count = result_spread.inject(:+) + expected_leftover_count
+  user_count = result_spread.sum + expected_leftover_count
   course_users = create_users_in_course(course, user_count, return_type: :record)
 
   # set up initial spread
diff --git a/spec/models/quizzes/quiz_statistics/item_analysis/summary_spec.rb b/spec/models/quizzes/quiz_statistics/item_analysis/summary_spec.rb
index 3c1cd93ec88..29524834a5c 100644
--- a/spec/models/quizzes/quiz_statistics/item_analysis/summary_spec.rb
+++ b/spec/models/quizzes/quiz_statistics/item_analysis/summary_spec.rb
@@ -48,7 +48,7 @@ describe Quizzes::QuizStatistics::ItemAnalysis::Summary do
 
       it "distributes the students accordingly" do
         buckets = summary.buckets
-        total = buckets.values.map(&:size).sum
+        total = buckets.values.sum(&:size)
         top, middle, bottom = buckets[:top].size / total.to_f, buckets[:middle].size / total.to_f, buckets[:bottom].size / total.to_f
 
         # because of the small sample size, this is slightly off, but close enough for gvt work
diff --git a/spec/models/quizzes/quiz_user_messager_spec_helper.rb b/spec/models/quizzes/quiz_user_messager_spec_helper.rb
index 734a22d7555..ae3b8f6d3f6 100644
--- a/spec/models/quizzes/quiz_user_messager_spec_helper.rb
+++ b/spec/models/quizzes/quiz_user_messager_spec_helper.rb
@@ -40,7 +40,7 @@ module Quizzes
 
     def recipient_messages(target_group)
       recipients = @finder.send("#{target_group}_students")
-      recipients.map(&:all_conversations).map(&:size).reduce(:+) || 0
+      recipients.map(&:all_conversations).sum(&:size)
     end
   end
 end
diff --git a/spec/selenium/grades/student_grades_page/gradebook_student_spec.rb b/spec/selenium/grades/student_grades_page/gradebook_student_spec.rb
index 04087783564..59db7f4250c 100644
--- a/spec/selenium/grades/student_grades_page/gradebook_student_spec.rb
+++ b/spec/selenium/grades/student_grades_page/gradebook_student_spec.rb
@@ -108,7 +108,7 @@ describe 'Student Gradebook' do
     means = []
     [0, 3, 6].each do |i|
       # the format below ensures that 18.0 is displayed as 18.
-      mean = format('%g' % (('%.2f' % (grades[i, 3].inject { |a, e| a + e }.to_f / 3))))
+      mean = format('%g' % (('%.2f' % (grades[i, 3].sum.to_f / 3))))
       means.push mean
     end