Is there a speed needed to achieve flight?
There may be an ultimate lower bound, about where the ratio of random air motion to forward motion is near 1:1. But that is an extremely slow speed.

I've seen competitive balsa-wood gliders, weighing just a few ounces, achieve flight from a rubber band, going less than 1 mph, or a slow walking speed.

If the craft is light enough, the wings are big enough, and a little air is moving over them, lift is generated.