So I might have found a bug...
The Swamp Clump says in it's description that it "Greatly Reduces Movement Speed", and it "Greatly Increases Life Regen" and I assume the other thing it does (which is worded very weirdly) is that it acts like a Cross Necklace as well.
Well, I can vouch for the Life Regen. It is definitely faster than Band of Regen, Charm of Myths, or the Heart of Atis.
But the Movement Speed, eh, no.
I've tried it with Vulture Wings and without Vulture Wings, and all four times (with Clump on, with Clump off), top speed was 34 in all 4 tests (Frostspark Boots), and acceleration was consistent with the 2 sets (obviously, with Vulture Wings you accelerate faster).
So basically, the Clump gives you the Life Regen (dunno about the iframes, haven't tested that yet), but no Movement Speed penalty whatsoever.