Minimal Reproduction of URL format slowdown Rails bug

If you load a JS view using the Accept header only (aka as an AJAX/UJS request) there is a mysterious ~10x slowdown. But if you add .js to the URL the speed is what you'd expect (with or without Accept header).

Very minimal repro

Check TestsController#test_nothing for a basic view, nothing dynamic, no nested partials. The slowdown is present but not so drastic.

Tortastic 🐢

curl 'http://localhost:3000/test_nothing' -H 'Accept: */*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript'

Started GET "/test_nothing" for 127.0.0.1 at 2017-09-01 16:35:33 +0100
Processing by TestsController#test_nothing as JS
  Rendering tests/test_nothing.js.erb
  Rendered tests/test_nothing.js.erb (0.3ms)
Completed 200 OK in 30ms (Views: 17.1ms)

Usual Rails speed 🐇

Note the .js on the URL - this is all that's changed yet the perf issue goes away.

curl 'http://localhost:3000/test_nothing.js' -H 'Accept: */*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript'

Started GET "/test_nothing.js" for 127.0.0.1 at 2017-09-01 16:36:44 +0100
Processing by TestsController#test_nothing as JS
  Rendering tests/test_nothing.js.erb
  Rendered tests/test_nothing.js.erb (0.3ms)
Completed 200 OK in 7ms (Views: 5.1ms)

Quite drastic repro

Check TestsController#test_nested for a view with nested partial in a loop to really demonstrate the issue. There appears to be a 10x delay for every partial rendered beyond the time Rails reports (you can see the partials don't appear to take any longer in the slow case, but they don't add up as the overall view takes way longer).

Hella slow 🛴

curl 'http://localhost:3000/test_nested' -H 'Accept: */*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript'

Started GET "/test_nested" for 127.0.0.1 at 2017-09-01 16:38:23 +0100
Processing by TestsController#test_nested as JS
  Rendering tests/test_nested.js.erb
  Rendered tests/_partial.html.erb (0.3ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.1ms)
  Rendered tests/_partial.html.erb (0.8ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.1ms)
  Rendered tests/_partial.html.erb (0.1ms)
  Rendered tests/test_nested.js.erb (129.4ms)
Completed 200 OK in 160ms (Views: 145.3ms)

Hella acceptable 🛵

curl 'http://localhost:3000/test_nested.js' -H 'Accept: */*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript'

Started GET "/test_nested.js" for 127.0.0.1 at 2017-09-01 16:39:11 +0100
Processing by TestsController#test_nested as JS
  Rendering tests/test_nested.js.erb
  Rendered tests/_partial.html.erb (0.3ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.0ms)
  Rendered tests/_partial.html.erb (0.1ms)
  Rendered tests/test_nested.js.erb (17.2ms)
Completed 200 OK in 24ms (Views: 22.0ms)

Doctor says just don't do that

The problem is Rails URL helpers don't generate the .js so this performance issue happens by default everywhere you do a remote JS call. Having to specify format everywhere would be a pain:

# I'd rather not have to specify `format: :js` everywhere.
link_to "I'm Sorry", podbay_doors_user_path(@dave, format: :js), remote: true

One could hack Rails UJS to stick a .js on the end of every URL it AJAXes automagically but I feel like there's something nefarious going on in the depths of Rails that should be fixed instead.

sfcgeorge / url-format-speed