Hello, I am running a early 2011 macbook pro (on el capitan) that was fine until a few weeks ago. I have constant lag/beachballing symbol,with playing itunes, login, i cant even type a document in pages without it freezing. Heck even loading this was a pain. I am posting for opinions on how to fix this, should i back up my must have things to an external and wipe it clean? reinstall osx? i have had it for years and it has never been like this. Thanks in advance for your help ps i foolishly turned on filevault and its 98% done but stuck on estimating time remaining..could that be a cause?