To see how fast your computer is compared to others you could use a benchmark like this one:
As an example my computer is in the 25th percentile, which means 75% of computers are faster than it.
You could show a recommended system requirements for your game like you see many titles do. I'd imagine they may test the game on a few computers first to get a better feel for it. Or you could just release the game and if you get a lot of complaints about performance you could attempt to optimize more.