User blog comment:Alartan/Swiftness or Mobility boots?/@comment-25003900-20150203162951/@comment-25254389-20150206010231

Thresh's ultimate quote

"Movement speed cannot be reduced below 110, and slows that would reduce it below 220 are modified. To put it another way: a reduction of 99% will not reduce you to 3 movement speed, but rather it will reduce you closer to 110 than a weaker slow would."

Boots of Swiftness arent really popular because they dont do more damage. The additional protection against slow effects was to increase their worth.

Any time Riot alters movement in the game Boots obviously take the largest hit. i remember when the boots of Swiftness granted over 100movement and champions could move over between 400 and 500.