I have an old comp (CPU Pentium II Celeron 400 MHz, 224 MB RAM)
running ham radio server in Debian 8. It works well in CLI, but very
slow after starting GUI. I wonder whether it would be worth to try
(if possible at all) to upgrade it to Debian 9. Any experience with
such old boxes?
It is not clear whether you are merely observing that it is slow with a
GUI running, or whether you would like to have a GUI, and are asking
for advice specifically there.
Assuming the latter, what do you want out of a GUI? An absolute minimal
GUI such as FVWM might serve you well enough, but I would not expect
miracles. Also consider a lightweight desktop such as XFCE. But I would
be surprised if that solution helped.