Not GP, but: I had replaced the battery on my M1 Max 6mo ago with an aftermarket part. On MacOS 26 that routinely gave about 5 hours of coding/browsing/active screen time (no audio or massive CPU load other than unit test runs).

Anecdotally, after updating to MacOS 27 2mo ago that time has been reduced by between a third and a half, doing the same kind of daily work on the exact same project, without major new apps installed or new always-open browser tabs.

I don’t know if it’s the OS’s fault directly or the fault of an app I use that interacts poorly with the new OS’s power management, but either one is damning. I’m not doing anything particularly weird.

Did you try downgrading?