When I click on an App, or some items in drop down menus, it takes quite a while to respond. The Apps bounce up and down, for a while, and drop down items have a little revolving wheel. Can anyone advise me, please? I am on the latest Yosemite, It's a 15" Mid 2012, laptop with 2.6GHz i7 processor. 4Gb memory and 1Tb HDD.