Calling .getCapability() on the FrozenColorizer.item is using up a large amount of processing time, which is especially noticeable with slipways since they call FrozenColorizer.nextColor() multiple times per tick per slipway. I don't know what the solution to this would be exactly, unfortunately; I'm going to be caching a large number of results for .nextColor() on startup for now, and selecting one of those at random.

Huh. Irritatingly I think that's a problem with capabilities themselves.

Have you checked on Fabric? i know CC's have a lot of finagling in the background to make them as fast as a normal fn call.