Yeah I would definitely agree that stackmat practice is essential, or as best as you can approximate it. I did an hour or two of practice before my first comp and is was harder than I expected to get used to it. Also, time your inspection, but don't use the whole 15 secs. In a comp, when they call 8, you should really start to go. 12 comes faster than you expect after that, and then you really have barely enough time to put the cube down and start the timer. If you're still holding the cube at 12, you're cutting it very close, and the best you can hope for is a panicked start

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