Thanks very much, again.
I have just tried using Guest, and that worked quite quickly. When I went back to my own user account, that also seemed to be much better than before, with very little delay.
I have Kaspersky Internet Security, and Adware Medic installed, as I read that they are good to have. I did have Norton 360, but uninstalled that, as I read that it is not as good as Kaspersky, however, a Norton reminder keeps coming up asking me to renew my subscription, and I can't get rid of that - I have searched Norton on Finder, but nothing appears, so can't get rid of this annoying reminder.
I am still trying to get to grips with Apple, after very many years of using Microsoft, and although there are some similarities, there are also many big differences.