I observe an increase in latency when opening a new file (prog mode, e.g., elisp). Then jit-spell starts up and I invoke consult-line immediately. Then filtering feels much slower than without jit-spell.

Unfortunately I cannot obtain a realistic profile - the jit-spell process filter doesn't appear prominently. However in top Emacs is taking much more resources with jit-spell enabled than without in this scenario. Without jit-spell Emacs creates about 15% load and with jit-spell enabled it creates 90% load. Something about jit-spell is inefficient. Maybe one really needs some additional delay such that the Emacs process filter becomes less costly.

(EDIT: See #1 (comment), where I initially mentioned that aspell appears prominently in top. This is solved by sug-mode=ultra, but Emacs itself is still too heavily loaded by jit-spell.)