The following is slow in Rails development:
The reason is that it is called often but usually returns nil. This is a problem with Ruby (http://redmine.ruby-lang.org/issues/4917), but Sass can avoid the problem by splitting up the line and returning early before trying to expand nil (yes it's a hack, but it avoids a performance bug).
real_file = find_real_file(dir, name)
return unless real_file
full_filename, syntax = real_file
Fix a Rails perf issue.