I've got a similar issue. Since iOs 11, my alarms actually go off 2 minutes AFTER they're supposed to. Weird, but it's been consistently wrong every morning.
And not only that, my snooze is now exactly 15 minutes instead of the expected 9 minutes. If that's a permanent change, I haven't seen it documented anywhere. In fact, I've read that it's still supposed to be 9 minutes. If I snooze twice, I'm 12 minutes late for work.
I'm updating to iOs 11.1.2, so we'll see if that fixes anything.