I came from Intego, Kaspersky, Norton, SOPHOS Pro and changed to Avast, because of its superior detection of virii.
To first of all to answer the ever re-appearing question 'I have a Mac, do I really need antivirus?': Yes, you do. It does not need to be a virus that affects OS X, you could inadvertendly cause an infection of others by you forwarding or sharing infected files. If you are in business and infect your customers, that is the sure way out of business.
I do not share the experience of any slow-downs by Avast at all. To the contrary, the use of cycles of Avast seems to be much less than other antivirus I have used over the years.
Apart from the ransomware shield and webshield, I particularly appreciate the e-mail shield of Avast, as we have been subject to a barrage of phishing and malware loaded e-mails, which all other antivirus did not detect. While I am certainly capable of identifying problematic e-mails, my associates may not. DIY also consumes precious time.
I don't share the experience of excessive invasiveness by Avast. In fact, I experience no invasiveness at all.
At the time of this writing, based on my experience, Avast antivirus is the best for Mac, in my opinion.