Lag is complicated. There are many factors in lag. Chess.com does everything it can to reduce lag.
It also measures the lag reported by the two sides and makes adjustments for the difference.
https://support.chess.com/article/423-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken
There are a number of things that you can do to reduce lag from your side.
https://support.chess.com/article/213-how-do-i-fix-my-disconnect-lag-issues
I have been very excited to finally reach 700 after almost 1 year. But after losing 5 games to lag, 3 in which I had winning positions, I'm back at 600. Chess.com needs to fix this problem with lag, or rather losing rating when you lag. Because lag is pretty much uncontrollable, losing rating would not be fair. Perhaps a solution would be to just not lose rating when you lag and disconnect.