Memoize Site#post_attr_hash (#7276)

Merge pull request 7276

lib/jekyll/site.rb

@@ -92,6 +92,7 @@ module Jekyll
       self.pages = []
       self.static_files = []
       self.data = {}
+      @post_attr_hash = {}
       @site_data = nil
       @collections = nil
       @docs_to_write = nil
@@ -232,6 +233,7 @@ module Jekyll
     def post_attr_hash(post_attr)
       # Build a hash map based on the specified post attribute ( post attr =>
       # array of posts ) then sort each array in reverse order.
+      @post_attr_hash[post_attr] ||= begin
         hash = Hash.new { |h, key| h[key] = [] }
         posts.docs.each do |p|
           p.data[post_attr]&.each { |t| hash[t] << p }
@@ -239,6 +241,7 @@ module Jekyll
         hash.each_value { |posts| posts.sort!.reverse! }
         hash
       end
+    end
 
     def tags
       post_attr_hash("tags")