It's true, it is open source and I am not a dev. So it is easy for me to complain about the poor performance in team games, even though I myself cannot put in the work to fix it.
Nevertheless, I have been asking myself recently: should I really be playing a game this laggy in 2024? Time out of one's day is lost in games where actual time is significantly longer (2x, for example) than game time.
WAY more than the unit rebalances or new units, I look forward to performance improvements like Vulcan. I also heard a27 will bring a way for the host to check if a player is causing lag due too just having a slow computer. So the host can choose not let that player in 4v4 games for example.
That is annoying. Probably what you are running into is the fact that a maximum of 200 units can be selected at a time. 2 things can help mitigate this now:
Hold alt while dragging the mouse to select military units only. Holding alt + y while dragging the mouse to select will select NON-military units only. Or,
Play games with a lower pop limit.
A bit of self critique: Some of the performance issues can be mitigated by choosing different options. We DON'T HAVE to play 4v4 200 pop team games. I think 3v3 is pretty interesting, actually. We can play lower pop limit games (albeit this effects the defensive structure vs unit balance).
Just because we have the freedom to choose grander options doesn't mean that it's the best choice.