Sorry for the delays. I finally got some time to test the snapshot (GregTech 6.14.11-43-g8aa1903f).
There’s still an FPS drop when holding down the right mouse button, although it may have improved slightly.
From my tests in the Spectre Dimension, it reduces to about 6 to 17 frames per second (from an average of about 100), which is a small increase from the previous 3-10.
I might try attaching JVisualVM to my Minecraft instance later to see if the sampler can identify where in the code the JVM is spending all of its time.