After comparing two projects side by side I figured that a different setting in screenspace reflections is at least partially to blame for the FPS drop. There seems to be a new SSR setting that adds reflections to rougher surfaces, which is quite performance-heavy. I can’t tell if this feature has been there in the previous versions, but it seems to be enabled in 4.26 by default. By typing *r.SSR.Quality 2 *into the console, I was able to get the old setting back and increase the performance a bit. It’s still nowhere where it was with an older engine version, but it’s a start.
Having looked into the GPU profiler a bit, it almost seems that the UI itself takes up ever so slightly more render time. I still haven’t been able to figure out what other factors are causing the performance drop, but I’ll keep sharing my findings.