Ground speed is affected by agility and your current weight. is not easily
noticed except with SoW or Selo's, since it magnifies the effect. I first
noticed the problem with my enchanter, a human, and a human cleric...
neither were encumbered, but my enchanter was running much faster than the
cleric. the only difference was his higher strength had him carrying more
pounds. My Druid wood elf and a wood elf ranger, same agility.... druid was
faster, less weight. my druid vrs one with a agility buff, the agi buff one
was slightly faster, but when I got my agi to 100, I could keep up with the
buffed one, (in the 120s). We did those while at the same weight.... done
at different weights, the heavy one always lagged behind.

Conclusions...
Weight is deciding factor... encumbrance only decreases your agility to slow
you down, but each pound slows you as well.
Agility is a factor, but only to 100... which is max groundspeed at 0 weight

I have 101 agility, SoW, and keep my weight around 55 pounds. I can run
circles around anyone in my guild, even the other shamans and druids.