I am using Minesweeper X instead of Windows minesweeper. Its more "professional" and can save replays among other things. Also, its impossible to produce fake times in it.

Expert: 67.02 sec
Intermediate: 28.23 sec (done under 15 on another computer)
Beginner: 5.90 sec


Also, win percentage is nothing, speed is the real indicator of skill. (Anyone can get 50% winrate on expert with practice, but not much more)