Hmm. I do see what you are talking about, however I haven’t really had an issue with playing fast. In fact, I usually play too fast (just not on the hard parts, which drives my teacher nuts… oops, lol). I also don’t have any memory of it usually being curled. I’m going to have to try to get someone else to take a picture while I play normally.
(Is this normal vibrato positioning? I was imagining something slightly different. But I guess that I don’t really see many other people do vibrato.)
I’m trying to imitate this position without the weight of the violin, and I cannot get my thumb to stay in a position remotely similar without buckling.