We saw memory go up when moving from 3.1 to 3.2, but we also removed compiling Ruby with jemalloc and used the defaults. We have seen performance increase by about 10% for our web applications. Our forking background processes run about the same. YJIT appears to perform for repeated requests – ie first run will not expose any benefit, ie your tests won't run faster.

[edit, the memory increase was small just a few percent IRC]