iMac running slow

I have a desktop which is just over 3 years old. Since I've installed Yosemite, it has been running slow. I get the rainbow wheel going round and round for several seconds every time I use Safari. It also happens when I open my own files and folders so it has nothing to do with my broadband speed, which is good. It seems to get a bit faster the longer I am using it. There is plenty of disc space, as I've used only about a quarter. Any ideas.