I'm about 200 hours into the game, and for the majority of that time have had virtually no unreasonable lag with the game on my PC. However, over the last month or so (it maybe started after buying both expansions?) I've been getting extremely bad lag - transitions between ships / the space center took sometimes more than a minute to load, loading a ship after clicking "launch" from the VAB can pause for more than a minute, and the game itself would sometimes get unplayably laggy (even causing UI glitches) while working with ships in space.
I have very few mods installed (Kerbal Engineer Redux 1.1.6.0, Kerbal Alarm Clock 3.10.0.0, and TriggerAU Flags 2.9.3.0; all installed via CKAN), but found a couple of old Reddit posts talking about lag and they mentioned that their only mods were with KER / KAC.
So I uninstalled KAC, and the lag went away entirely. Reinstalled KAC, and after not much gameplay the lag was back (though not as bad). Investigated logs, and sure enough: the pauses were occurring when KerbalAlarmClock is logging events. See timestamp LOG 00:22:04.213 in this log, where the game pauses for more than a minute and a half between these two items:
Any thoughts on things I could do or check? I can't play the game without KAC