User blog comment:Alartan/Swiftness or Mobility boots?/@comment-25003900-20150203162951/@comment-4022742-20150205115552

Except for the fact that it's not at all how Boots of Swiftness works. Tenacity reduces duration while Slow Resist reduces potency. The 99% slow from Thresh only becomes 75% with swiftness.

Seeing as you can have tenacity from other items too, that means you could have the best of both worlds.