Actually I have two issues
a) The dramatic slowdown if Windows Explorer
b) The fact that synchronisation , which in my case is > 800GB is going at a pace so slow, under 1 file per second, on a VERY (and I stress VERY) fast fibre line, means it would take more than months to complete.
So NOTHING has been suggested to correct the Windows Explorer
But I was advised to manually start Insync every time with this command line
(obviously replace my userid mbennett with yours)
My initial testing showed NO IMPROVEMENT
So in the interim I have gone back to the native google drive client from Google. And whilst that does not work either, I'm making slow progress there at least and the explorer is NOT slowed down.