Opening and browsing the book causes my FPS drop to about 5-10 fps from a capped 60fps. The frakk?

Shift-F3 to open the profiler. Send screenshots of how it's like with and without the book open.

Here we go https://imgur.com/a/AC0NU . Did a couple takes.

In the menu press 1 to open gameRenderer (or the other numbers to open other categories, 0 to go back), try to see if there's anything way higher when you have the book open than when you don't and screenshot it.

https://imgur.com/a/7v5uA , unfortunately the "unspecific" column keeps increasing while the book is open. Up to 90%.

If it's unspecified it's not gui, so it can't be the book itself. Try removing mods until it stops happening. It's possible one is trying to render something over it. I can't reproduce with only botania.