Lightroom 6.0 - sssllllllllllooooooooowwwwwwwwww

I have Lightroom 6 installed on my PC and it has worked very well until a few days ago.  It has become very slow particularly the 'Import' function which takes several mintues (>10mins) to respond to selecting the "Import" source - it often shows 'Not Responding' so one has to be extremely patient (and go and make a coffee) while waiting for the source menu to appear.

Do any of the experts have any suggestions? 

Ken M