Stibbons

I have a question about the way speed capping works if you are slowed.

Lets say I'm slotted to go faster than the cap with SJ. The speed I have over the cap wouldn't be used under normal circumstances. What if I get hit by a slow that reduces movement by 25%? Will the extra speed I have kick in to prevent me from getting as slowed as I would normally? Or would I just move at 75% of the cap?