Just now it started updating Pathfinder: kingmaker. Nothing big, 169 Mb patch. Downloaded fast enough and then 'Applying' began. 4 hours later it crawled to 96%. And during that time my PC is just unusable, CPU load at 100% and stutters to even open Start menu. Is it a problem at my end or just a result of some wierd optimization by gog team? To compare Steam does 200 Mb patches in 3 minutes total.

That's not a Galaxy problem. That's a Unity problem (and possibly a dev-optimization problem as well). Unity-based games are the only ones where I encounter such ridiculously long patch-jobs.