Unfortunately Sophos does slow down the system. I've tested it. And the marketshare theory, that there will be more Mac malware because there are more Macs, is nonsense. There was more malware for Macs in the 1990s when Mac marketshare was under 3% than there is now. And Sophos today can do nothing to defend against any new malware which might turn up. It just can't, the devs at Sophos don't have crystal balls and can't tell the future. If it makes you feel better, by all means keep using it. Just know that you're not doing much real good. Do carry on, now.