Why is this?

Booting from a pretty static (no new items added to startup), defragmented daily, OS partition on a mechanical SATA II hard disk it takes w10 some weird random bootup times. Sometimes it boots taking 35s and sometimes is taking 70s. Sometimes I get different numbers in the same day. I check them using a custom view for event viewer. This is driving me nuts. I remember that when I had w7, boot times, were pretty similar.

There isn't to be a common pattern to this. It seems totally random.

Any thoughts?