It seems likely that variance in lag may be a factor. However, that thing you call lag is the response time of chess com’s servers combined with the effects of many Internet hops outside of Chess com’s control. Those many hops must affect the hops of lichess signals as much as they do the chess.com signals.

The combination of all those factors is measured by ping.
I think it would be interesting to keep track of the ping results daily and figure out if there is indeed a significant correlation with your rating changes.
Guys, hello.
I play at chess.com for very long time.
I have three bars out of 4 on connection indicator at the beginning of the game and ALWAYS have 3 bars from the middle and at the end.
And only now I realized that something really strange is happening to me.
1) Usually (most of the time) I am play in lag mode having 2050 elo maximum.
By lag mode I mean following.
At the beginning of the game I usually have very good connection, making 5-10 moves in 1 second (with pre moved). Sometime even when completely pre-moving like that site counts -2 seconds for my timer. Then I kind of start to think for each move and each move consumes more time than I expect. Feeling is like site is eating my time off. Due to this I try to pre move more and do stupid mistakes and lose.
I paid attention that many times players from India just own me on time.
They kind of having handicap.
2) Sometimes site decides to do a favor to me (good mode) and bullet feels very light and I am going to 2150 range easily with 0 effort. Such good mode don't last too long - maybe a day or two, going back to lag mode. And I stay in lag mode for weeks.
The Lichess site behaves always the same at any time. I have there 2150 ELO always.
When I lag on chess.com or I don't lag at chess.com. ALWAYS the same.
I have some nostalgic feelings for chess.com. I appreciate chess.com team fighting cheaters, returning me ELO that I lose to cheaters (whom I submit frequently).
But I find myself more and more playing on Lichess.
Do you have the same issue?
Something similar?
Thanks a lot.