I must admit that I don’t know. One thing to exclude would be stochastic variation (e.g. %timeit gives you a standard deviation, so you can imagine error bars for the measurement), but I would not see 30ms->60ms doing that.
The other part is that you need a really stable environment to get reliable benchmarking, maybe something we did here changed something w.r.t. other things going on.
(But maybe @ptrblck knows something.)
Fun anecdote: A long time ago, I briefly enabled remote access to the GPU I used for benchmarking for one of fellow PyTorch devs because I somehow had a much more stable timing environment then them.
Best regards
Thomas