Problem solved. I had not been clearing the saved tabs and it had gotten up into the hundreds. I'm not sure how so many accumulated but when I cleared them all was working well again.

I thought this solved my problem and indeed the browser now works much more quickly, but still I have the following problem. When I select the square box in the bottom row, second from right, the one that has a number in it representing the number of open tabs, and reveal the open tabs, most of them are blank. Only a few are populated with tiny images of the web pages to which they refer. In the past I recall that they all had images. Any suggestion on how to fix that?