Most likely stuff with that kind of time constant, is couple-meter wires from PS to board, scope probe, uh USB if this thing is connected, etc. I'd expect bypass caps to be slower, and immediate signal quality things to be faster (~ns, and probably not visible on the scope at all, or at least very well).
It also keeps ringing for a while, which suggests a high Q, low loss -- if it's bouncing between cables and boxes, it's doing so for a fairly long time. In that case, a ferrite bead somewhere will definitely cut down on the duration, though not so much the peak amplitude right after the edge (except if it's CM up the probe, in which case it may).
Tim