User blog comment:RoznosicielCiast/Other stuff about LoL, this time from other author/@comment-70.61.75.234-20130311191956

I thought it was common knowledge, but there is a softcap to movement speed. After you reach a certain movespeed point, the bonuses are reduced by a certain percentage.

Straight from the wiki page on movement speed:

When the raw movement speed is greater than 415, there are two soft caps applied: The two ranges have their reductions applied at the same time before
 * The raw speed between 415 and 490 gets multiplied by 80%.
 * The raw speed over 490 gets multiplied by 50%.

Since Mob boots give +105 bonus OOC, and the lowest base movespeed is 325, it will always have SOME reduction of it's bonus.