Minecraft 1.16.5
Forge 36.2.2
Feywild 1.3.1
Better Foliage 2.7.0 & 2.7.1

Note: I have also turned off in the configs the following for added performance: Config>Feywild>client.json5 - Tree Particles: "false" (and yes I tried true as well)

With both BF & Feywild installed (and supporting lib mods, etc.) the FPS drops when in Feywild biomes. Normal FPS on default settings was around 60 and they drop to as low as 20 at times though average around 30-40. This may not seem like much but add in a few other key mods and your FPS isn't that playable anymore in Feywild biomes. I tried with Dynamic Surroundings and it behaves similarly to BF in respect to the FPS drops but other mods like Astral Sorcery have no effect. I can state that the leaves are not actual leaf blocks but that's about all I know.