Stop booting so often. ESPECIALLY if you have a laptop.
Get suspend to ram to work. Good for several days unplugged.
Get Hibernate to disk to work. Good forever, needs a swap file/disk
Stop shutting down, and just close that lid. Its easier on the machine too.
I agree with everything @Chrysostomus said except compiling your own kernel. You will never get those hours back. Especially not on a rolling release.
There are also some things you can tweak in systemd if you look at
ystemd analyse blame
(or the more informative option )
systemd-analyze plot >plot.svg && firefox --new-window plot.svg
With that you can find things that can be re-arranged by making local copies of systemd scripts in /etc/systemd/system/ (which are used in preference to the ones distributed in the distro).
For instance, I know that the chances of me wanting to print something the INSTANT I log in are slim to none. My traveling laptop is unlikely to be near my printers much of the time anyway.
I know that cups takes a long time to initialize, so I fiddle with the cups.service to push it out to way after login. (like 5 minutes), and I make sure nothing depends on it being up. I don't wait for cups any more.
Same thing with modem manager.
But honestly, I seldom reboot. I've closed my laptop lid, driven from Washington state to Arizona, opened it again and resumed work.