I have a 2011 i7 3.4 iMac with 16GB of RAM. In the last year or so it seems to be getting slower. I don't want to re-install all my apps so I haven't bothered to do a clean OS install. I've migrated every OS from Snow Leopard to Yosemite. If I format and install Yosemite fresh, but restore everything backup after, will that negate any benefit of a fresh install?