This topic has come up before, and I can provide my input. But need to ask you some important questions first:
1. What exact Mac model are you using?
2. What exact Mac OS are you running?
3. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You can actually do quite a lot of disk cleanup on your own, and there are some excellent products available (both free and commercial) that can help you with that.
4. Are you making backups to an external device? That task is so, so
critical. If you are doing that, what software do you use for that task?
5. How much space is on your internal drive?
Now, some folks here have had issues having such virus software as Avast, Norton, etc. running. Basically, such software is always "active", ie, lurking in the background. What I have found successful are the following steps/tasks:
1. Keep my Macs "lean, mean, and clean". As I mentioned above, one can do a good amount of disk cleanup on their own.
2. Be careful about which sites I visit and/or download software from. One of the best practices, at least for applications, is to download such software/updates from
their site only.
3. DEFINITELY do not click on suspicious/unknown links in EMails.
4. There are two excellent programs you can use. One is Malwarebytes, available from here:
https://www.malwarebytes.com/mwb-download/
As you can see, there is a free version. It is good at finding some malware, and adware.
The other one is ClamXav, available from here:
https://www.clamxav.com/
You can use it in trial mode, and it is very good. I eventually purchased it. It does a more extensive job than Malwarebytes.
The thing about both of those programs is that you run them when you want to, and thus they are not "lurking" in the background, thereby not "hogging" any resources.
