I couldn't find a thread like this so I've decided to start this one.
Whenever I would face very large amounts of ordnance (close to 10 squadrons of voidcraft fighting each other combined with normal batteries firing, especially lasers - tau light cruiser spam seems to do that too) the game is capable of going down with FPS really hard (usually from 60-70 down to ~20). I'm not running some top rig, mine is like ~5 years old but I did recently upgrade my GPU to gtx1060.
Ork ships in large numbers when hitting red button seem to be fairly taxing too, possibly due to smoke they generate behind them.
Setting the game to lower details does not seem to do much (I've tried going from all high down to low-ish), my PC handled large battles in Stellaris fairly well but this game sometimes seems to be too much.

I can't say much more, I just hope it gets optimized better as I love this game and have always supported it.

If win 10 look for power setting in pc settings not in game setting i have a 6 gig 1060 and i had a power setting in a energy saver mode so card was under powerd if you need help there are vids and forms on how to fix also inclued processer type and speed and ram size. Just video card does not work aslo include 1060 type card info like name and ram on card
Also stelliars as a referance is like refering a go cart is as fast as a drag racer.