Stacking movement buffs is additive, meaning at a base speed of 6.0 if you have +10% and +15% you end up with 7.5 speed (6.0 * 1.15) instead of 7.6 (6.0 * 1.15 * 1.1).
However, slows are calculated in after movement buffs which results in them being significantly more effective than speed buffs.
Example:
+25% movement speed from buffs.
-25% movement speed from debuffs.
Base speed of 6.0
6.0 * 1.25 = 7.5 buffed speed.
7.5 * .75 = 5.6 debuffed speed.
As you can see above, having +25% movement speed doesn't cancel out -25% movement slow. In order to cancel out a 25% movement speed debuff you need 33% movement speed buff. Basically, the higher your movement speed the more effective movement speed debuffs become. 25% debuff on 6.0 slows them by 1.5 points but the same 25% debuff on 10.0 slows them by 2.5 points.
To make matters even worse, movement speed debuffs stack additively (not multiplicitively) which means if you are buffed to 8.0 speed and take a 25% debuff (dropping you 2 points to 6.0) and then you take another 25% debuff you end up dropping *another* 2 points to 4.0, instead of dropping 1.5 points to 4.5.
Basically what this comes down to is you can *never* beat movement speed debuffs with opposing movement speed buffs. Not only are debuffs inherently more powerful than buffs (as shown above) they are far more common:
-25% favor item
-25% gloves
-25% Oak ability
-25% UB ability
-15% TB ability
-10% TB ability
-50% Rook ability
-25% Queen ability
-25% Erebus ability
-25% Regulus ability
-25% Regulus ability
+15% TB ability
+25% Sedna ability
+25% UB ability
+10% boots
+15% favor item
+25% usable
+15% boots
+50% boots proc
