If crafting a stack by shift clicking in the Crafting Terminal output slot, it results in massive lag spikes
I've tried FastBench but it is not improving this behavior in this case.

I hope you can improve the crafting code to reduce the lags, it can be very annoying on servers with some peoples online at the same time.

Maybe it would be possible to outsource the task in a separate thread to keep the server running while the code is running. Or limit the task to not overload the system
Not exactly sure how, but it should be improved please.

Same, im playing ATM 8 and i added this mod to the pack, its doing the exact same thing, I give 16BG of ram to my singleplayer modpack.

You should probably attach your modlist.