I have a relatively light modpack (120 mods) put together for testing purposes. The first thing I noticed in a creative test was that flying around for world gen was super stutter inducing compared to normal. I started turning off mods one at a time and found that Bewitchment seems to be the cause.

Without Bewitchment loaded, world gen is super smooth (at least as far as MC world gen can be); even over oceans or around large extreme hills or savannah plateaus. However with Bewitchment loaded, generating ocean or large scale biomes like those causes what I can only describe as stutter like my computer were a low-end PC trying to run a top-end AAA game.

Here are two videos for comparison:

Without Bewitchment

With Bewitchment

My computer specs:

Intel Core i7-7700HQ @2.8GHz
Nvidia Quadro P5000 w/ 16 GB DDR5 VRAM
16 GB System Ram
500 GB SSD