As a notorious Minesweeper speedrunner I find the animation uncover animation when middle clicking to be a little slow as I find myself waiting it for it to finish for a second or so.
I've done quite a few iterations of the speed at which they get uncovered, and it's definitely tricky to nail down. I've tried to find a middle ground between something that's rewarding/satisfying (so should take 1-2 seconds, long enough to be enjoyed), but also doesn't block you (so should be instant). And that animation would need to very different if it's revealing 10 tiles or less, or 500+. I realize this still isn't quite right.
So thank you for saying that, I'll try and improve it.
But in short, when you uncover the first tile of this wave, at each frame I uncover only 1/60 of the total. So if you've uncovered 600 tiles, 10 should be uncovered on each frame, and it should be done in 1 second if your display is at 60FPS.
But if you uncover 60 tiles only, it will uncover 1 per frame, and that can look pretty bland. Could it be what you see? If that's the case, I believe (with a good 60% certainty!) that I mostly need to revisit the method when it's for a small number of tiles.