That's just a failure in the dramatic camera work and time freeze filming. The glass shouldn't have been spinning end over end as quickly or nearly at all at those slowed frame rates and Barry's hand should have remained in the shot the whole time. Just inches away. Poor film editing and scene layout choices.
Yeah. Bad visual effects work. Honestly though, what they're trying to portray is nearly impossible to actually fit on a single screen image. Well, not and make it look at all dramatic. Assuming the glass is traveling at say 50 mph, and he's traveling at 1400 mph (I think those are the numbers tossed out earlier, and if not, they're good enough for a simple example), then he's actually going 28 times faster than the glass. Even doing the typical super slo-mo type shot (and yes, the glass should not be spinning at all if you've slowed it down that much), the time it takes for him to fail to grab the glass (versus succeed with miliseconds to spare) is so small relative to the distance you could fit in the shot that it just doesn't work visually. He's catching up to the glass so fast that if you had any time to show his hand in the shot, he should have been able to grab the glass. If you have any distance between the glass and the victim in front of the glass, he should have had time to grab the glass.
Using those numbers as samples, if we put him 5 feet from the target at the beginning of the shot, and he's traveling 28 times faster than the glass, the glass needs to be 2.14 inches from the target for it to reach the target in the same amount of time he can. Which doesn't make for a very exciting shot, no matter how much you slow time. If he's ever seen close to the glass (like his hand within a foot for example), the glass should be so close to the target that it's impossible for the audience to tell it hasn't hit her yet, for him to still fail to catch it in time. Again, that makes for a terrible shot. Oh. And technically, he'd still reach the glass first since that's the equivalent distance the glass would travel while he traveled 5 feet. But he's 2.14 inches closer to the glass, right? Anything with him farther or the glass closer doesn't give us a shot at all, much less any possibility of showing his hand being "close" to the glass while still failing to catch it prior to impact.
So they go for drama rather than accuracy. Annoying, but I can see why they'd do it that way. Drama says we need to show the dangerous object moving towards the helpless victim. But the physics of the situation are such that any movement we could readily and clearly show would be too much for the hero to have failed to save said victim. Same reason why the glass is spinning in the shot. You need that to show the mostly physics ignorant audience that these are shards of glass, and they are moving, and we know this because they are spinning, and there's blurred background, and all the other stock visual tricks that film folks use to show us these things. Ultimately, I chalk this up to the same sort of thing as why you can hear the Enterprise's engines as it flies by in space. It's about creating a more satisfying image for the audience, not about portraying things 100% accurately.