Same way there are items that increase your speed, there are items that make you slower, but only 2:
- Depth Calcei: -5 speed
- Fur Boots: -6 speed
There are spells aswell that can decrease your speed:
- Sharpshooter: "Utito tempo san". It decreases the speed 70%.
Net conections can slow you down too. Playing in a server far away from your country (as me that play in NA and live in spain, I usualy play with 130 ms, when in european I got over 30), or your computer graphic (fps), the better it is the more fps you got, so casting spells or spam in the screen won't make you lag.
And as zupakode says if your keyboard delay is high, you will be slow.