So I was thinking since the passive slow scales off of crit and frozen mallet is an on-hit item, would a full crit Ashe with Frozen Mallet and Guinsoo's be able to slow for 100%?
I would test it myself but I can't and I just had to know.
Nope, slows cap. Thresh's 99% slow isn't 99% for example.
The lowest movement speed possible is 110.
The only champion that could reach 100% slow was Nami, but I think that got patched out. Not that you'd ever see it though because of how much AP you'd need to make it happen.
https://www.youtube.com/watch?v=HvpZhXOlU2U
vs Yi (who dances faster based on movement speed)
