2% doesn't sound like that much of a difference, but when you're dealing with superspeed and lives are at stake, it's apparently a hell of a lot. It's the difference between stopping that shard, or it injuring Iris. It could be the difference between defeating Zoom, or receiving yet another beating. The reason that he was getting slower is that Harry Wells finally put his Turtle Power to good (well, bad) use, finding a way to drain his speed when Barry was in the Flash suit, and then hands it to Zoom, who promptly injects himself with it. However, Barry now knows about it, and he's gonna need all the speed he can get to defeat Zoom. He's been working on getting faster and developing more powers, but is he gonna be able to get that speed back?