I know it's not particularly believable at this point in time, but Yatsufasa is right. If anyone was to get frustrated at you for not being fast, I think they've forgotten that they're getting free art. Chill and try to forget about it. Shouldering the grinding guilt of not working makes it very difficult to recharge, which ironically makes it take longer to get back in the saddle.
Yatsufusa
My advice would be to not worry about something like that.
I think the artist should always take priority over the art.