Try troubleshooting Safari extensions and third party plugins.
From the Safari menu bar click Safari > Preferences then select the Extensions tab. Turn that OFF, quit and relaunch Safari to test. If that helped, turn one extension on then quit and relaunch Safari to test until you find the incompatible extension then click uninstall.
If it's not an extensions issue, try troubleshooting third party plug-ins.
Back to Safari > Preferences. This time select the Security tab. Deselect: Allow all other plug-ins. Quit and relaunch Safari to test.
If that made a difference, instructions for troubleshooting plugins here.
Sorry for the delay in responding, but thanks for the help!
I've now tested as suggested by disabling extensions, and plugins, with re-starts in between. Unfortunately neither made any difference.
I've not worried to list the plugins as disabling them all completely made no difference.
Any other ideas, please? This is driving me nuts...
This fixed it for me.
Not 100% smooth, but MUCH better. Before it was unusable. Offending files seemed to be: