Lower the Battle Size in the options; that's the biggest-single optimization you can make to save CPU.
Secondly, play Vanilla, or with very light mods enabled; Nexerelin in particular is a giant CPU hog, largely due to having too many fleets and too many Markets in the sim. Please bear in mind that, to be frank, the modders have largely been working with the game's current framework and issues and not a lot of effort's been spent on making them optimized; the last time we had something like an optimized large-scale mod was about two years ago, and it's been abandoned because the game's fundamentals have been a moving target
Thirdly, consider using the
FX Mod; that helps quite a bit, in terms of toning down CPU use due to certain types of special effects, especially ship engines and trails.
Fourthly, the
AI Overhaul mod, while imperfect, can reduce the CPU hit from ship and weapon AIs (weapons on Autofire are actually a substantial hit, and it improves that load a lot). But it's definitely not playing quite as well as the Vanilla AI, as I have limited time-budgets to work on it, so you'll have to bear with some imperfections
Fifthly, please bear in mind that this is basically the work of one guy and that he's largely concentrating on reaching the final sets of features. There are a bunch of optimization areas that can and will get looked at as the game reaches its final form, I suspect