Okay, I've got some more info on Purge.
"Movement Update Frequency" (let's call it MUF for short) is supposed to be a number between 1 and 10.
When MUF = 10, then the unit will start regaining his speed after Duration * 0.4 seconds (this is the minimum: any MUF higher than 10 will be Duration * 0.4).
When MUF = 8, then the unit will start regaining his speed after Duration * 0.5 seconds.
When MUF = 3, then the unit will start regaining his speed after Duration * 0.667 seconds.
MUF = 1 is obviously Duration * 1 seconds, so the unit will regain his full movement speed only at the end of the spell.
Now the problem is that I cannot find a good formula here, because some values completely break it (MUF = 4 is sometimes the same as MUF = 8, and MUF = 5 is sometimes less than MUF = 8).
I'm going to guess that is because of the way it adds the movement speed (I told you it increases it in percentages, which is correct, except that it isn't the complete truth, which is slightly more difficult and I'm not always good at explaining things).