Does a USB 2.0 device connected to a USB 3.0 hub slow down the hub to 2.0 speeds?

Short answer: No.

Long answer:

I stumbled across the answer to my question in a comment to an answer for a seemingly unrelated question. It turns out USB 2.0 and USB 3.0 are physically segregated in the wiring, so plugging USB 2.0 devices into an USB 3.0 hub won't affect the performance of USB 3.0 devices in the way that I was wondering.