Yes both of my kids do it, first and second grade. I was just talking to a teacher friend about it, I'm against the idea of timing it because I absolutlely hated doing that in school back in the day. I still stink at math today, but i don't want my kids to stink, so I do want them to know these facts so well it's like breathing for them. So we practice, the second grader is supposed to do it every night but we only do every other, First grader isn't required to practice at home, but she is super competative and often asks to practice.
I think that is one of my complaints about it, It's supposed to be them challenging themselves to better their time, but All the kids in the class seem to know what level the others are one and are super competative about it.
I will say, my kids struggled more in the beginning and it has progressively gotten easier or less stressful for them. The beginning of the subtraction was like a piece of cake after having done all the additions.
If your son is that stressed out, have a discussion with the teacher and ask her, her opinion on what aspect of it he needs help with. I'm sure it depends on the teacher but with out me asking they have adjusted my son's goals when he was hitting a wall and the frustration was causing him to shut down a bit. Maybe just knocking the goal back a smidge will be enough to ease up the pressure.
Another idea might be to give him 2 mins to reach his goal one night and 1.45 the next, and 1.3 the next and work it like that. IF it's the speed that is the issue. I think we just needed more practice writing small and fast.
my kids already knew all the facts or could figure them out by counting back or on, so for them it was just practicing speed.
If it's a problem of not knowing the facts, knock out the easy ones they do know, like +1, and doubles and just concentrate on slowly un timed learning the facts.
Good luck, I know i'm still conflicted about the whole idea.