In the past week or so my previews have slowed right down. Can take a minute or so before internal or external previews pop up and during that time CoffeCup is locked to use.

You most likely have a script or multiple scripts running on your page that are slowing everything down. Try commenting out or removing all the scripts you have running and see if that fixes it. If so then add the scripts back in one at a time until you narrow down which one it is (most likely a social networks or google script). Then keep that script off until you want to upload and check the site. It's a pain, but pretty much any editor you use will do this so you may as well get used to it.

Especially Google map scripts make it very slow.

Google can suck the electrodes from your pc sometimes.

Absolutely fixed it!

Many thanks for your fast, accurate and comprehensive advice. Google analytics script was the culprit.

Glad you got it fixed. Thanks for posting back.