One thing that got my minesweeper game times down was to never actually mark the mines as such, just click the unturned squares you know aren't mines.
For reference, the best of the best is Kamil Murański. There is only a 0.4 second difference between his NF (No Flag) time and his normal expert world record:
https://www.youtube.com/watch?v=GrZCWx0fnfc (flagging)
https://www.youtube.com/watch?v=8bsLwGf-vUE (no flagging)