Performance

I think it would be really cool if Lichess added a thing where the computer asses your strength in a game and tell you what rating you played like in the analysis of postgame. Just a suggestion. This may help people set goals.

As in ‘You played at the standard of 1700’ in the analysis?

That would be great

Yea. Like a computer analyzes your game and says something like

Performance: 2178 or something like that

Only great if it actually means something. Probably not enough info to make such an assessment.

A popular rival site has this as part of their game analysis feature. Pretty depressing to be told that you played like you are 850 rated.

But performance doesn't exist like that. Only performance rating exists which isn't actually the same as what we want.

I've found that when playing in marathons, arenas, etc, it will give you an average performance rating, like what you suggested. To be taken into consideration for implementation as you suggested it, I think you need to submit a request on github.

The average performance rating in an arena is simply based on your win-draw-loss record against opponents and what their ratings are. It does not rely on engine analysis at all.

Engines are not going to be very good at assigning an Elo play strength to a single game, and I doubt that however chess.com does it has any particular statistical validity.