I assume that Boots of Swiftness reduce the magnitude of slows by 25% (As in a 40% slow would be reduced to a 30% slow), but the wording is a little vague.

I did some testing on this a week or so ago.

1. Slow resistance is additive, meaning boots+passive=40% slow resistance.
2. It works like how you said. Meaning with 40% slow resistance, 30% slow=18%, 40% slow= 24%, 50%=30%, 60%=36%.

Excellent. This makes boots of swiftness awesome on characters who get kited.

like singed needs more CC reduction :P