I have several issues with around 50 custom fields. When doing updates to these issues, I identified poor performance on the @editable_custom_field_values@ function being called to check if current user has access to the custom fields.

Here is the code for this method (in @app/model/issue.rb@):

  # Returns the custom_field_values that can be edited by the given user
  def editable_custom_field_values(user=nil)
    visible_custom_field_values(user).reject do |value|
      read_only_attribute_names(user).include?(value.custom_field_id.to_s)
    end
  end

Here, it seems that @read_only_attribute_names(user)@ is recomputed for each @visible_custom_field_values(user)@ without caching, which will slow down the action if the issue has many custom fields associated (only if user is provided). I have applied a simple patch (I store the result of @read_only_attribute_names(user)@ before the loop):

  def editable_custom_field_values(user=nil)
    read_only_attr_names_array = read_only_attribute_names(user)
    visible_custom_field_values(user).reject do |value|
      read_only_attr_names_array.include?(value.custom_field_id.to_s)
    end
  end

Now, some results computed for an admin user, for approx 100 issues with various custom fields: Before:

  2.2.2 :001 > puts Benchmark.measure { u = User.find(88); Issue.last(100).to_a.map{|i| i.editable_custom_field_values(u)} }
  2.2.2 :002 > 102.260000   0.710000 102.970000 (103.480136)

After:

  2.2.2 :001 > puts Benchmark.measure { u = User.find(88); Issue.last(100).to_a.map{|i| i.editable_custom_field_values(u)} }
  2.2.2 :002 > 5.070000   0.090000   5.160000 (  5.198187)

Environment:
  Redmine version                3.3.0.stable
  Ruby version                   2.2.2-p95 (2015-04-13) [x86_64-linux]
  Rails version                  4.2.6
  Environment                    development
  Database adapter               Mysql2