You should go here and download and install the The Sins Optimization Project Rebellion (TSOPR) mod. It will work for straight-forward Rebellion and may work on some smaller mods that don't rework textures, meshes and particles of things already in the game.
This is hands down, the best way to reduce lag in the standard game.
https://forums.sinsofasolarempire.com/428154
If you really like one of the available large mods and that's all you run you will have conflicts until those mods adopt the improvements from TSOPR but otherwise will have no problems.
