# UNDF: UNDF-2026-000000876
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1332,15 +1332,16 @@
   # Returns true if this issue blocks the other issue, otherwise returns false
   def blocks?(other)
-    all = [self]
+    all = Set.new([self])	# Use Set for O(1) membership instead of Array O(N)
     last = [self]
     while last.any?
       current =
         last.map do |i|
           i.relations_from.where(:relation_type => IssueRelation::TYPE_BLOCKS).map(&:issue_to)
         end.flatten.uniq
-      current -= last
-      current -= all
+      current.reject! { |c| all.include?(c) }	# O(1) per element with Set
       return true if current.include?(other)

       last = current
-      all += last
+      all.merge(last)
     end
     false
   end