Writers have been conspicuously vague about her exact top speed. The closest we've come is when they were in a de-briefing after a fight against another team, and realizing their intel was wrong. Hawksmoor said something to the affect of "His top speed is supposed to be around 500 mph. He'd have to be going a lot faster than that to keep up with Swift."
I may be wrong about 500, but it was in that ballpark. She's also shown reflexes to match, and tagged a zombie-speedster once (ripping him in half, actually) in one of the Authority's annuals.
She's also broken the sound barrier easily, and has traveled to remote parts of the globe in a few hours or less. I'd say 1000 mph is a low-end estimate, since it's likely quite a bit above that. If she were going balls to the wall, I'd guess she could maybe approach Mach 5 or so. You'd know more about Hawkman than I to say if his speeds are equal to that.