What command do you use to reboot from the terminal?
If you use the following:
qdbus org.kde.ksmserver /KSMServer logout 0 1 1
Which is the proper way to log out of Plasma session, it shouldn’t make any difference in speed.
If you just use reboot, that’s obviously going to be faster, because it does save the current session, it does not log you out, it also does not run any logout scripts which you may or may not have set up to run on logging out of the Plasma session.
If you don’t want Plasma to remember the current session (open all programs you had opened when you logged out), you can turn that off in the System Settings. This might shorten the logout time for a second or two.