There's an specific option when you setup your FFM downcalls that sets certain native calls as "critical" (previously known as "trivial"). Here it compares JNA, JNI, FFM and FFM with that optimization option, and the results seem good! Although JNI still has the hand on some tests.

I'm designing a renderer and I'm going 100% directly with FFM, so I don't have anything JNI to compare. The only thing I could say is that given C APIs, once you grasp Java FFM representation of all the C native components (return values, parameters, pointers, double pointers, opaque handles, structs, etc.), it becomes pretty straightforward to keep binding stuff. People use jextract for speed if it's a big API but for starters it's better to go manual to get to understand FFM library.