I am getting massive 2-second lag spikes every 3 seconds while playing FTB Monster, including in menus and the inventory screen.
I am not playing on a flash drive, and turning the graphics settings to minimum levels has not reduced the spikes one bit.
Despite this, the debug data shows that the majority of memory use is switching between gameRenderer and render, and either the entities or the terrain.
This is a newly generated world (ATG worldgen) that I'm just exploring on creative. I'm not even adding or breaking blocks. I get the spikes whether I'm moving to new terrain or wandering old ones, so it's not waiting for the chunk to generate. I'm on peaceful, so there are no more than maybe 10 passive vanilla animals rendered at any one time.
Optifine is not compatible at all with FTB Monster (crashes on startup), and I get a good 70 fps when using fancy settings and normal render distance while I'm playing vanilla.
I didn't get these spikes when I was playing FTB Horizons (though a reduced 25 fps), so it's probably due to one of the mods in Monster.
Any ideas which one could be doing this?