we have the same issue since version8, every RU takes extensive long time, first time insttall is fast, but RU installation takes very long times.
I have rebooted a test server once after 6 days.
Somehow I am happy to see this, as so far symantec told me it's just our environmnet and not their software.
I have spent weeks in investigating this issue, and AV Software seems to have an impact, but I even got this issue on servers which did not have AV Software installed and defender turned off, so there must be another cause too.