It definitely was, however I need to stress that I'm talking about the stand-alone AV program, not 360 or the Internet Security Suite. The NIS has been a nightmare for some of my customers. The majority of my customers, on their business systems, use the Symantec Endpoint Protection software managed from their server. It's very low impact.
As a side story to the early comment about not seeing any slowdown using the various AV software, if there is no slowdown it may not be doing its job. But the performance degradation is relative to the base system performance and other applications that may be running. Using ancient terminology, every file that's touched, by default, is scanned. It is checked against a database of millions of possible virus matches so it's obvious there will be some impact.
Several years ago one of my customers called to tell me he had lost Internet access. I walked him through some possible solutions but nothing I could do over the phone worked. When I got on-site the first thing I noticed is that he had Norton Internet Security installed, he neglected to tell me he installed it. I told him I would solve his problem by uninstalling the Internet Security portion of the software and leave the anti virus part. I did, and he was blown away by the performance improvement. I have found over the years that people pay less attention to the performance loss then they do to a potential performance gain. It's human nature I guess.
Any time I clone a drive I turn the anti virus off first. The speed improvement is obvious.