back up the watch often so the restore process is easier (if you aren't backing up already).
if you can reproduce the freeze issue or if it's still freezing, then immediately report error to wear app > menu > contact us > error report (you need samsung phone for this).
I started having the same problem recently where my Watch 4 Classic would become slow and unresponsive. The only way to fix it is to reboot the watch. This has happened to me 4 times now and is getting annoying.
I don't know if it is due to the latest update Samsung applied to it or some app that causes it.
What I seem to notice when it happens is when I get text message(s) coming in and the watch stalls and becomes extremely slow and unresponsive to screen swipes or touches.
I hope Samsung can fix this problem with a new firmware software update!