Even basic redstone contraptions are causing severe lag and framerate drops. I typically get well over 100 FPS (redstone_clock_disabled.png). To duplicate, I created a simple redstone clock that toggles a number of redstone torches and dust. While active (redstone_clock_enabled.png), this causes my framerate to drop to about 25 FPS (using the bundled javaw.exe 1.8.0_25 my framerate drops to 5 FPS).

I think this may be a lighting issue, however. I tested a redstone contraption that does not create lighting updates with a piston and rapidly flipping the lever causes no lag. If I add a redstone lamp, the lag spikes return (redstone_lamp.gif). Ninja Edit: I also get a lag spike when placing and item in a furnace and it fires up.

This may be related to the issue reported here: MC-124086 which I believe has been erroneously marked DUPLICATE, as this bug is not present in Minecraft version 1.12.0.