Is AE2 much laggier than RS? When I tried to switch to AE2 system my TPS rapidly increased, sometimes even more than 40-70TPS. Maybe it is just a correlation, but is there a way to check what causes lagging in my world?

TPS should max at 20, and higher is better. Are you talking about something else?

AE2 is much less laggy then RS for servers.

When I type '/forge tps' and is is below 20 game performance is good, but when it Goes over 20-40 delay starts (lags)

You're likely looking at the mean tick time part of the output, any mean tick time above 50ms will decrease the TPS (ticks per second). As for RS, that's always been laggy on servers, to the point of crashing them.
example below of a /forge tps output
[01:46:27] [Server thread/INFO] [minecraft/DedicatedServer]: Overall: Mean tick time: 121.359 ms. Mean TPS: 8.240

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.