I have a decent enough PC (8 GB of RAM, i5 6600K, Radeon RX 570) that should be able to handle Feed The Beast: Endeavour fine, but my FPS jumps around between 5 to 10 FPS. What can I do about this?
Try setting Mipmap Levels to OFF in instead of 4 which the default value is. This increased my performance from around 5 to 10 FPS to around 80 FPS.
I'm not sure why this helps exactly, but looking around the Internet I'm far from the only one to have had this resolve FPS problems in Feed The Beast on otherwise powerful hardware.
Especially in modded Minecraft, reducing particle counts can improve CPU performance dramatically. Even though Minecraft is now a AAA title, it still uses the CPU to calculate particle physics, and with a single core! As if this is not bad enough, there is no culling that's done with this, meaning that all particles are taking physics computations all the time.
In short, reducing particles to minimal, or better none will get rid of a lot of client-side calculations, and as a result boost your FPS by a significant margin.