perf: reduce number of calls for student and teacher survey items in response rate calculators

app/models/student_response_rate_calculator.rb

@@ -49,7 +49,7 @@ class StudentResponseRateCalculator < ResponseRateCalculator
         value < threshold
       end
 
-      ssi = @subcategory.survey_items.student_survey_items.map(&:id)
+      ssi = @subcategory.student_survey_items.map(&:id)
       grade_array = Array.new(ssi.length, grade)
 
       memo[grade] = si.slice(*grade_array.zip(ssi))

app/models/subcategory.rb

@@ -109,4 +109,12 @@ class Subcategory < ActiveRecord::Base
     Zones.new(watch_low_benchmark:, growth_low_benchmark:,
               approval_low_benchmark:, ideal_low_benchmark:).zone_for_score(score)
   end
+
+  def student_survey_items
+    @student_survey_items ||= survey_items.student_survey_items
+  end
+
+  def teacher_survey_items
+    @teacher_survey_items ||= survey_items.teacher_survey_items
+  end
 end

app/models/teacher_response_rate_calculator.rb

@@ -12,7 +12,7 @@ class TeacherResponseRateCalculator < ResponseRateCalculator
   def survey_items_with_sufficient_responses
     @survey_items_with_sufficient_responses ||= SurveyItemResponse.teacher_survey_items_with_sufficient_responses(
       school:, academic_year:
-    ).slice(*@subcategory.survey_items.teacher_survey_items.map(&:id))
+    ).slice(*@subcategory.teacher_survey_items.map(&:id))
   end
 
   def response_count