It's an old game that can only use a single core.
So maybe try to set a priority to your strongest core.

Also try toggling the HW mouse acceleration in game settings to another setting. Close any other app using the core game runs on. Avoid building huge stacks of small ships and if enemy has those hunt them down. Google on EAW stuttering/lag for more. Best is actually to use an older CPU with strong single-core performance but fewer cores, that said I run on a 5600x I think t is called and I rarely have issues.

The bigger the map and the more factions, the more it'll do this and especially if you take your time. The AI loves cranking out units from every planet they own at the same time. Now imagine this multiplied by the number of factions. Generally the biggest offender would be whoever controls the most worlds; it's almost always going to be the GE in the early game but it can be anyone with 20-30 worlds or more. Try focusing them down for a bit and you'll see it lag less.

Thank you! But I'm also facing problems on the spaceships battles

The same limitations apply in combat unfortunately. The game tracks individual hardpoints, fighters and projectiles and then the ships themselves. Larger battles lag quite noticeably and if your PC doesn't have strong single core CPU performance framerate in battles does suffer.