In the world of software, it is quite common to find free or demo versions that are essentially useless. The full functionality becomes available only when you buy it. Not so with Avira. The free version is pretty effective in protecting your system from viruses on a real-time basis. Yes, there are many value-additions(like Ransomware protection) in the paid version but the free version is pretty good.
Yes, it does slow your system down, especially during booting. That’s because it looks for updates and then downloads and installs them. Also, its real time protection is always on - so yes, there is some pressure on your system memory. The problem is more pronounced in Windows 7 systems. far, far less in Windows 10/11 systems, especially if your RAM is 8 GB.
Overall, I would say Avira has played a key role in keeping my laptops, desktops and pen drives virus-free for over seven years now. I am very happy with its performance.