I'm not sure, honestly.
Charitably, we could assume it's just from removing Google and various carriers background apps meant to improve my experience.
Uncharitably, I have my suspicions. For the last five or so versions of Android something always seemed to be using processor cycles and battery when I wasn't actually doing much with my phone.
But I never saw evidence of usage data exfiltration via Google apps - at least after I turned off the related optional settings.
In any case, switching to GrapheneOs was a startling and pleasant speed boost for me, whatever the real root cause.