We upgraded to 2.6.1 about a week ago and switched to using the new(ish) parallel(ish) garbage collector. I still can't tell what the impact has been.

Claude Code (which is a wizard at analyzing log files but also, I fear, an incorrigible curve-fitter) insisted that it was a real breakthrough and an excellent choice! On the other hand there was a major slowdown last night, ending in SBCL dying from heap exhaustion. I haven't had a chance to dig into that yet.