My computer is old.  Bought in 2007, it has Vista Ultimate as the operating system and an Intel Core 2 Duo E6600 processor.

It had an Nvidia GeForce 7600GT and that was having problems because 2 of the capacitors were bad.  I had an Nvidia 9600 GSO given to me by a generous person and I installed that.  Next I removed all free antivirus programs and installed Webroot.  It turned out that AVG was not actually uninstalled and I had to get a special program to remove it. 

After doing that and using CCleaner to judiciously remove other defunct dll's and remove all traces of AVG my computer now boots and shuts down about 3 times faster.  It also runs much better.

I purchase a new HP laptop and used AVG along with some other free software.  After installing those programs my new computer took about 3 times longer to start and shut down.  I did about the same thing on it as my old computer and now it starts in about 1/2 minute and shuts down in under 20 seconds.

I've always felt the free antivirus programs were sufficient and I still do.  Webroot found nothing different than AVG free.  However my computer is faster in all three ways, boot, running applications and shutdown.   I feel it's worth the money as the $29 total for the 2 computers isn't much but the time factor is worth it to me.

If you don't care about the start and shutdown time I think AVG is fine, whatever constraints it puts on everyday computing probably won't be noticed on the latest computers with fast CPU's and 8GB's of memory.

Your results may vary this is just my experience with 2 very different computers.