Have MBPro purchased in Spring of 2009. Was running Snow Leopard (10.6.5) and "upgraded" to El Capitan (10.11). Performance is so poor, want to revert to Snow Leopard. Have copied pertinent files to portable drive. Have original div for Snow Leopard 10.6.3. Is reboot with that dvd, all that's needed? Will El Cap osx be deleted or overwritten completely? Any help most appreciated; first time on MacRumors!