Previews very slow in 14.0.1

On a MacBook Pro using OS 15.1 I have around 500,000 images in my catalogue. All RAW files kept on external raid drives. Around 100,000 have smart previews. All running smoothly until I updated to 14.0.1. Now smart previews load very slowly and some normal previews are not showing at all. 

Ive tried trashing my Previews.lrdata and all the normal things.

Exporting and all other functions normal.

Any advice please ?