Hoopy you've made my year! So, glad to hear some feedback!
The 550 objects sounds like an anomaly. I've never seen an object count that large during playtesting - 250ish is the norm. It's difficult to predict an exact number because new ships (tharglets & vipers) are constantly being launched and destroyed. My machine - Black MacBook, Intel Core 2 Duo, 2G Ram - usually drops from 100 fps to around 50 fps, but there's no shuttering. The optimized OXP has no scripts that run after exiting witchspace.
As for difficulty: the answer is it depends. This is war after all, and its unpredictable. The biggest difference maker is how many cruisers and battleships the Thargoids have in their fleet (assuming you have Thargorn Threat installed). If the Thargoids take out your larger cruisers early on, victory is almost impossible.
There are 3 types of missions which are randomly assigned at systems throughout the galaxies - intercepting an incoming Thargoid fleet, cleaning out Thargoids from an already invaded system, and destroying a pirate fleet. The latter mission is Reserve forces only (no large cruisers).
meet the fleet and beat up a fairly small number of thargoids
This shouldn't typically be the case. The missions should have you facing off against 25 Thargoid warships with the possibility of launching 150+ Tharglets! The invasion missions have the Thargoids spread throughout the system, but the intercept mission sends them all in at once - see pics:
I very much welcome any feedback on making the missions better/more fun/more desirable. Not fishing for compliments - I just spent tons of time scripting this and want people to enjoy it
