Slows being partially mitigated was PoP-era and beyond. Everything is 100% or 0% here.
Few mobs are flagged as immune to slow. Most that are immune to slow are completely immune to magic (again, pretty rare). I think there are a handful of mobs here immune to changes in attack speed, but not MR.
Here's an approximate list:
http://webcache.googleusercontent.co...ests/slows.htm
Also worth noting is that slow is not a disease yet.