@Tresset I've never used NearInfinity before so I don't know. In EEKeeper or Shadowkeeper there's a field on the Characteristics tab where you can change it.
I tried searching for a solution to my problem online. Apparently the disappearing familiar is not an uncommon problem. I found this one guy who said he got the familiar working properly again when he tried releasing it indoors. I just tried it and it worked, but for some reason it has to be inside a building, a cave doesn't work.
[EDIT] Actually, it doesn't have to be indoors. It just has to be on a different map from where your familiar died.
I think you'll all laugh when I tell you the quick and dirty solution I used to get the cat moving at normal speed.
I gave it "The Paws of the Cheetah".
Except for this: I put the cat in my backpack for a while, and in my next session I found the Paws of the Cheetah in my party leader's inventory. I didn't make the connection at first. Couldn't remember Consoling one in or adding one with Keeper, so I decided to investigate. I loaded a previous save and my cat was there. I pulled him into my pack, and lo and behold, "Your party has gained an item." Yup, Paws of the Cheetah. He will also give them to you if you ask for any items he has pickpocketed. And now, of course, if you deploy him again he's half-speed again.
I'd edited the fix so that the boots won't be dropped by the cat when it is placed in your backpack.
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[EDIT] Actually, it doesn't have to be indoors. It just has to be on a different map from where your familiar died.