Multicoloured circle of death since Yosemite

I have had my Macbook Pro MacBook Pro (13-inch, Early 2011) 2.3 GHz Intel Core i5 Memory 4 GB 1333 MHz DDR3 for about 3 years now and it has gradually got a bit slower in responding. I have a very fast internet connection (speedtest 5 mins ago 79Mbs download) and i use chrome. The internet can be very slow on my machine with entries I have typed taking a few seconds to appear on screen. Other functions such as loading programmes also produces the dreaded multicoloured circle. It is very frustrating. It seems to have got worse since I installed yosemite. I was always pretty savvy about Windows, but I'm still floundering a bit with OSX. Any help much appreciated. I'd buy some software if it would help speed things up. I've even thought about swapping to a solid state hard drive.