IIRC its capped at 95% and has no diminishing returns.
Just a tip, dont add small amounts to you if your merc has more, because it takes each total individually, so your merc might do 50%, and you 20, whenever you hit, it cancels the mercs 50%slow and starts your 20%, which is this case would be bad. So if your melee, either you or your merc carry all the slow gear.
In addition there are independent caps on all the things affected by slowing.
You can't ever move slower than 1 y/s. Different monsters have different normal walking speeds so this 1 y/s represents a different % lower cap, but it is typically around 25% of normal walking speed (which means you probably can't slow them down the full 90%).
Melee attacks can't be slowed down below 15% of the base animation rate for the attack. This is more complicated to explain, but the gist is you may not get your full 90% slowing there either unless the monster attacked at higher than normal speed to begin with.
Cast rate also has a lower cap but I don't recall what it is (it is probably pretty similar to attacking, as both are to do with changing the speed of animations).
Slow actually effectively has the opposite of diminishing returns. It applies linearly, but if you consider the effect of 10% more slow at normal speed compared to 10% more slow when you are already at 50% speed, the 10% makes more difference at the lower speed.