Welcome to the forum!
The mag springs are pretty stout when new, and I load new magazines to capacity and let them sit for a week or so before I use them. I also clean and then lock the slide open for the same period of time.
You didn't say if you were trying to load the magazines with 16 rounds into the gun with the slide closed, but if so, that's pretty much a common issue with a new gun/new magazines. Try loading the magazine into the gun with the slide open, or download it a round or two on a closed slide.
I'm not sure what could be causing the mag(s) to drop out on rapid fire, but the standard checks would be:
1) Are you hitting the mag release button with your thumb/finger(s) as your grip shifts under rapid fire?
2) Do both/all mags drop out under rapid fire?
3) As previously mentioned, do the mags drop out under rapid fire when downloaded (i.e., 15 rounds or less)?
It may simply be a matter of checking your grip. If only one magazine drops out, then it may be a problem with that magazine. If it happens with both/all mags, your gun/mags may just need a little "break in" time. Are you an experienced shooter, or is this your first semi-auto?
Good luck, and I hope you find a solution. Be sure to clean and lube your pistol...may not be the problem, but it can't hurt.