The very oldest (early 80s) Kermit implementations were limited to 96-byte packets. Kermit gained support for 4K and 9K jumbo frames, sliding windows, etc., and was generally faster than ZModem -- when there was a modern implementation. But a lot of BBSs (and also client-side communication software) did their own from-scratch Kermit implementation, which was tiny packets and no sliding windows, so it was slow. There's some discussion of it here: https://www.kermitproject.org/misconceptions.html#notslow

This is what I remember. Kermit hated latency, and dial up modems had that in spades. It was much slower than Z-Modem for pushing files around.