impossible to slow the FPS processing. Ashley can't/won't apply an advanced FPS controller. Ashley has a strong opinion that there should never be a reason to slow the renderer, but there may be limitations within the browser environment itself.
The best you can do is swap the your own logic of thinking to do this. Instead of running your game logic update nativly you can in stead update your game logic using X seconds.
so if you know that 60fps is apx every 0.16 seconds. Then
On Every 0.32 seconds(every 30fps)
-- do logic
however this will require you to do a lot of custom logic and redifine the numerous behaviours move X persecond values.
so then you could do something along the lines
var fps_slow = 0.32
Every fps_slow + (numberofsprites on scree / 40)
that way you can try to mimic the slower aspects of having more objects on the screen.
The other option as some one mentioned. Use a loop to lock the. personally though I don't like that and it feels sloppy.
Burvey
It wasn't working the way expected because it was a quick hacky version. Ashley mentioned that he feels it's not worth writing a more complicated FPS controller.