When debuffing a mob and standing still, there is a visual bug that occurs where the mob seems to run at you as if slowed.
Afterwards the mob will teleport to you, because they are not really slowed; it just appears that way. Unfortunately, I cannot show this in my gifs as they can only be 7 seconds long.
Here are three gifs, and a comparison to the Old Client:
Here, the mobs appear slowed if you debuff them - tested with three different debuffs:
Intimidation Debuff - https://i.gyazo.com/98ef8882f9eb8bad156f900dde3cbd06.mp4
Shake Debuff - https://i.gyazo.com/095df45f0fea87c7eb262adcb775304b.mp4
Yield Debuff - https://gyazo.com/f40d6758c9b57d413f6bad982d066def
Here, you can see the mob approaches me at the correct speed:
Tested with Shake Debuff - https://i.gyazo.com/153ea0f929a25bf1211db29cc552c03a.mp4