ShaderLib isn't free on the CPU: it's a 10-20% performance hit to the CPU in battle. No performance loss in campaign unless you don't have enough VRAM to load up all the textures (can happen if you're using lots of factions - especially Scy - and have under 1GB), in which case your performance will tank.
Currently, vanilla Starsector campaign's main performance hits are hyperspace storm calculations, fleet AI, and rendering fleets. These are all CPU hits, not GPU hits (yes, even the rendering is more of a CPU than a GPU hit). If you're in hyperspace with a lot of fleets on screen, that's basically the worst-case scenario.
I'm having an issue: moderate spikes of lag in the campaign, which I can deal with, but massive lag in battles. My computer is a piece of crap, so I've disabled shaderlib, but something else is going on that is taking up huge amounts of resources.
This happens with battles with just 3 frigates (mine plus 2 enemy). With vanilla I can handle the 80 ship mega battles without any lag or framerate issues. I've also noticed increased stuttering during "preloading" of the main menu, but I just figured that was due to the larger number of textures being loaded.
Any idea what could be causing this or advice? As with most I very much enjoy this mod and would love to play it (thanks as always for the hard work of making it!).
How much VRAM do you have? If you run out, performance will
die. Some mods (SS+, Scy) use a lot by default due to the amount of graphics assets they load.