Hi Im not sure I noticed it as much in previous versions of El Capitan but since 10.11.4, I occasionally get the beachball appear in Safari, but it does not seem to screw up anything, but just show itself for a few seconds and then disappear after I click on another link or tab. Also today I had Apple Mail and Safari open and the beachball appeared, this time however it really screwed things up, I could not force quit from the Apple Menu and no key combo to force quit would work and I could not log out or restart. In the end I was forced to power off/on from iMac power button. It's quite annoying such things happen on our iMac, its normally very snappy. I must note the above happens very rarely, I'm guessing its just El Capitan bugs?