I was wondering, when we see the Flash do everyday things, like ordering coffee or having a conversation, he seems to do it at "regular" speed. Is this because he can turn his speed "on and off" or is he essentially doing it super, super slow?
The Flash can turn his speed on and off.
As shown in The Flash Vol 2 #30, the Wally West Flash could sit down in a movie theatre with his girlfriend, and perceive a movie running at the same speed as normal people. But when he subconsciously turned his speed on, in response to the sensation of a speeding bullet beginning to press against the back of his neck (a gunman at the back of the theatre had just begun to unleash a hail of bullets into the audience), everything and everyone around him suddenly froze in place from his perspective.
At first, he was confused as to why everything and everyone had suddenly stopped moving, but soon realised what had happened when he noticed the bullet seemingly frozen in midair, just behind his neck (technically, it wasn't strictly frozen, just moving very slowly).
In explaining what had occurred, he specifically noted that his speed had turned "on":
WALLY WEST: I must've turned my speed on sub-consciously when I first felt the bullet touch my neck.
After intercepting all but one of the bullets fired into the audience, and disarming the gunman, he then turned his speed back off again momentarily, so he could taunt the gunman before knocking him out. His exact words in this case were:
WALLY WEST: All I have to do to start time again is will myself to stop.
(That's when he realised he'd overlooked one of the bullets, because the one he missed immediately took out the exit sign over one of the doors.)
When you think about it, it really has to work this way, otherwise the Flash being able to converse with normal people -- in such a way that both sides can understand what is being said -- would be either impossible, or at best, impractical. I mean, even if it were somehow possible for the Flash to understand words being spoken at an excruciatingly slow pace from his perspective, imagine the inordinate amount of patience it would take to stand there and listen for that long, and to speak in turn at the same, excruciatingly slow pace.
From a writer's perspective, it's an absolute no-brainer that you'd want a character like this to be able to turn their speed on and off as necessary.