Retextured heads in CIT Resewn leads to substantial fps drops.

CIT Resewn 1.0.1+1.18.2

In the image below, all these items (excluding two) are heads that have been retextured.

Running the resourcepack under Optifine yields ~ 80 fps in this inventory, while under CIT Resewn yields ~ 20 fps.

I am not sure as to what the difference is between Optifine and Resewn's implementation of rendering retextured heads, but it seems that Resewn's implementation is not efficient enough to handle heads.

Can you send the resourcepack and a list of head items that cause this?

Absolutely

Slimefun-Beautified-Light_v1.0.zip

This is a Slimefun resourcepack so you'd need Slimefun to test the heads. As for a list I dont have one on hand and cannot get one at the moment.

The pack is just very expensive to run because of the amount of checks the CITs perform.
Luckily I predicted such a pack would exist so there's a fix!

Assuming you have both Mod Menu and Cloth Config.
Go to Esc/Main Menu > Mods > CIT Resewn > Configure and raise the cache time to whatever works best for you, after raising it on my end I noticed the fps drops were gone when I opened an inventory with a lot of retextured heads.

This will however make certain CITs take a tiny bit longer to update on the item but it shouldn't be that bad depending on how high the cache time is set.