General, Miami has typically been rated the slowest conditions on tour for hard courts. Balls fly through the air slower due to ever present humidity, compared to Indian Wells dry desert air. Courts don't have quite as high a bounce as Indian Wells normally does, but it's not low bouncing by any means.
But I did hear that the new layer of paint they put on this year may not be as gritty as in the past, which might help make things a bit faster this year. But I haven't had a chance to see any matches or get word from any players thus far, so I can't confirm yet.
Respectfully,
masterclass