I just installed Mojave on a 2011 iMac. It is running 10.14.3 and has taken an Apple update without bricking. Websites/internet/wifi/bluetooth/USB all run smoothly. HOWEVER, the system is VERY slow now compared to when it was running High Sierra. Objects moved across the screen create acid trail duplicates that persist for some time. Any idea what could be gimping the system? Please advise.