Thanks for all the information.
Not doing any disk cleanup/maintenance/repairs is not good. If you had been doing it, then trying to narrow down what is causing your issue would be easier, and most likely, more successful.
Also, not doing any backups is troublesome also. There will come a time when you will have an issue that will require a backup. It's not a question of "If", but "When".
About the only potential "problematic" third party app is Sophos Anti-Virus. As long as you are careful, there is really no need for an Anti-Virus program on your Mac. You should get rid of it (and all its "pieces"). The best way to do that is to first download and install the excellent, free third party app AppCleaner. You can get it from here:
https://freemacsoft.net/appcleaner/
Given that you are using Yosemite, you need to get Version 3.4, the first one on the list. When you launch it, just drop the icon for Sophos (should be in your Applications folder) onto the appropriate spot on AppCleaner. It will then find all the pieces associated with Sophos, along of course with the app itself.
I don't use an Apple mouse, preferring ones made by Logitech. But I assume you have the latest drivers for it via Yosemite.
I see you have VMWare Fusion. Do you have a Windows partition? And if so, does the mouse work fine there?
Finally, you should try another browser. Either Google Chrome:
https://www.google.com/chrome/browser/desktop/
or Opera:
http://www.opera.com/computer/mac
will be fine. I use Chrome as my primary one (it's much faster than Safari), but I am also using Opera more and more (also faster than Safari).