[Byrjun]

It seems like there are some mobs where MR spells like Cripple can land easily, but slows are always resisted.

Feels a bit too consistent to be random luck.

This would not be a classic mechanic. A mob is either MR immune or not. This is a good time to also bring up mobs like Gorgalosk and Deep (monk epic) that are flagged as slow immune which is a Luclin+ mechanic.

Edit: Mobs that do this are reportedly LTK and Vindicator, probably others. These mobs should be completely MR immune from my past experience and guides on the internet, so not only is the cripple/slow thing a very unclassic mechanic, cripple shouldn't even be landing on these mobs.