OK, I bit the bullet and installed Mojave.
Nearly every app was slow to open in the first instance but picked up the second or third time.
Topaz Studio the first time locked up completely and I had to force quit. Subsequently it opened but then this 'Syncing Content...' business which it does every time it opens (very annoyingly) took ages and occasionally on subsequent restarts locked up again. On the occasions that it opened, the "Syncing Content..." failed and returned the message "there was a problem syncing content".
It's a problem that has been happening for some time, I think it has been reported before, but with Mojave it appears to be a bigger problem. I don't know exactly what it is doing but even if it fails it appears to make no difference to normal operation of TS, so why is it there? And, if it is necessary, why is it not working correctly?