I'm playing the Skyblock 4 modpack, and with FastFurnace enabled, my slab furnaces (from the Slab Machines mod) don't operate at the same speed. One seems to work normally, but the second one seems to hang/pause randomly, but quite often, resulting in it actually processing input at roughly half speed.

Disabling only FastFurnace fixes the issue. I haven't been able to test it with a normal furnace yet but figured I should let you know. I'm guessing it's an issue with this mod and not Slab Machines because, when this mod is disabled, they perform as expected.

Why would it be an issue with this mod? This mod has no mod integration of any sort, nor any code that modifies either vanilla or mod code.