I was away from FAF since the first Steam link requirement. Now I tried and succeeded setting up FAF with GOG. 
The issue:
Everything lags. Some things more than others. Some things lag/stutter more than last time I tried playing some years ago.
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1st, the client app, it's all right, I guess. No worse than trying to use the modern gmail webapp. Though, I can't fathom what could the client app possibly need so much system resources for.
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2nd, the game lobby, it takes several seconds longer to load than vanilla (which already takes a bit). I thought FAF's less flashy lobby UI would need less draining. But I guess this stems mostly from the game's underlying inefficiencies.
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3rd, the in game lag and occasional stutter is noticeably worse than vanilla's, especially the in-game UI - for example if, on a factory I press a unit build button (eg. an Engineer) multiple times in quick succession, about every second or third press is ignored, unlike in the game's vanilla version.
I already run the game on nearly the lowest video settings (the only thing I'm not compromising on is running full screen). I've ran through the forums and the mod vault but at a glance I haven't found anything that would make a noticeable difference.
Although my system is rather old (was barely middle end in 2010), it can run vanilla FA even with mods just fine with little to no lag (mostly the main menu & lobby stutters a bit). I have no idea what to try.
I looked around for what to do but I am not knowledgeable enough to play around with recording, encoding, playback, etc. timings settings, which seems to be the cause for the super-jumpy playback.