perf: memoize function when calculating response rates

app/models/student_response_rate_calculator.rb

@@ -28,16 +28,19 @@ class StudentResponseRateCalculator < ResponseRateCalculator
   end
 
   def survey_items_with_sufficient_responses(grade:)
+    @survey_items_with_sufficient_responses ||= Hash.new do |memo, grade|
       threshold = 10
       quarter_of_grade = enrollment_by_grade[grade] / 4
       threshold = threshold > quarter_of_grade ? quarter_of_grade : threshold
-    SurveyItem.joins('inner join survey_item_responses on  survey_item_responses.survey_item_id = survey_items.id')
+      memo[grade] = SurveyItem.joins('inner join survey_item_responses on  survey_item_responses.survey_item_id = survey_items.id')
                               .student_survey_items
                               .where("survey_item_responses.school": school, "survey_item_responses.academic_year": academic_year, "survey_item_responses.grade": grade, "survey_item_responses.survey_item_id": subcategory.survey_items.student_survey_items)
                               .group('survey_items.id')
                               .having("count(*) >= #{threshold}")
                               .count
     end
+    @survey_items_with_sufficient_responses[grade]
+  end
 
   def survey_item_count(grade:)
     survey_items_with_sufficient_responses(grade:).count