Just-in-time (JIT) compilation can greatly speed up the emulated Amiga 4000 (It should not be used with cycle-exact 68000/68020 emulation).
There isn’t an “official” FS-UAE option to enable JIT yet, but you can use the following UAE option override to enable it:
uae_cachesize = 8192
In FS-UAE launcher, you can open Custom Configuration from the menu and type it in there.
Note: JIT currently only works on the 32-bit builds of FS-UAE for x86. Also, to use JIT on Windows, you need to have at least FS-UAE version 2.3.14.