I have a mob farm that spawns mobs really fast (in a few seconds(modded)) and when I turn off the lights it doesn't start spawning mobs with the PR lamps. But if I use vanilla Minecraft lamps it works just fine.

Which version are you using? There was a bug with the lamps that caused this, but it was fixed a long time ago.

ProjectRed-1.7.10-4.7.0pre5.89

Fixed via 2a850ab