There are two kinds of anti-malware software: 1) the kind that promises 24/7 protection by running in the background all the time. This kind of software tends to cause at least as many problems as the malware it claims to protect against. It also fails to deliver what it promises. 2) the kind that you run manually to see if your computer has picked up anything. I use only the second variety and in the 35 years I’ve been using a Mac I’ve yet to find any malware on my computer. The user is the weakest link in security and all anti-malware software does is offer the user a false sense of security.
Unfortunately my employer requires that all computers used on its network have anti-malware software of the first variety so my work systems do have Sophos on them. The program doesn’t appear to have a huge negative impact on my computer but given that my personal computers have no protection and I’ve never picked up a baddy I cannot speak to the effectiveness of the program.