Well, I was having the same problem, was looking for solution and decided to make an account here and write about it, since I'm quite a fan of DF.
So this has happened to me two times.. I created demigod human adventurer... After some time of playing I decided I should become a vampire + necromancer since no resting really is great. So after a while (I'm not sure if it was after becoming vampire or necromancer my speed decreased from over 1800 to 1000 or 800).
Now, my second time (same with demigod human), same thing happened, but this time I was paying a lot more attention. I found a necromancer tower just near start, rushed there (saved before it), picked the book and read it. Speed didn't change at all, picking heavy stuff only lowered my speed from 2057 to 2000 and reduced it by 50 furthermore... After some time of quick traveling my speed decreased to 1305 or something. I was not on ground neither was I swimming, sneaking etc... Dropped everything in my inventory, my speed only increased to 1400.
So the possible options probably are:
- as soon as you become a vampire your speed lowers by ridiculous amount,
- same with necromancer, speed drastically decreases,
- or maybe just playing demigod person triggers this.
I really want to know if this is just a feature and I'm not informed about this matter enough, or a possible bug. People, if you have similar problems write them down, so we can compare a little, or if anyone knows the reason, do tell.
cya Emptiness