Avast does use more resources, but it performs pretty well compared to most out there, protection-wise.
From what I've seen first-hand, with people having a number of infections picked up on the Avast scans that it failed to originally block with its live protection, and overall seeing a number of Avast users have far too much crapware quietly installed on their computers while running avast... I would have to disagree. Of course, end-user competence is always something that needs to be taken into account; however, what I've seen leads me to believe Avast lets too many things slide that it should have easily prevented.
I'd hate to use a solo example, but take it with a grain of salt, as it represents the average computer user I run into (not to the same extremes, but you'd be surprised how incompetent the people I fix computers for really are). My dad used Avast and Ad-Aware for computer security; and his computer problems have gone on for many years. He's had more viruses/trojans than I can keep track of, and has the worst internet habits I've ever
witnessed. (Keep in mind, this man is too stubborn to go with a new computer, or at least upgrade his current XP Pro/Pentium 4 machine to Win7). I recently recommended Bitdefender be installed on his system, and he did it... sort of. He removed Avast, removed Ad-Aware, and attempted to install Bitdefender multiple times; each time failed, and one time he received a BSOD. I went through the computer for about two hours, removing crapware, cleaning up background processes, updating certain drivers, doing a virus scan, all routine stuff; and I managed to get his computer running pretty close to stock again. Hours later, he tells me Bitdefender quit working entirely, he could not revive it, and he's reinstalled Avast. For many years I've avoided security software, and have had far less trouble than my dad has in a single week. He was idling with more resources than your typical Win7 install, and he's running XP; yet he claims his computer "works."
Forgive the slight rant, but this does have some relevance. All of the problems my dad has experienced started while he was using Avast and Ad-Aware, and now he's left with "no choice" but to continue using the same software that made his 500 MB memory usage idle north of 1.5 GB. While under the "protection" of Avast, he managed to have his machine go from near stock performance to slower than any other computer we've ever owned. What does he do on his machine? He uses Chrome and/or Firefox (and rarely updates either), browses the internet for mundane things (mostly using Yahoo, unfortunately), checks his email in his browser, and rarely even uses it to watch linked/embedded YouTube videos. The biggest activity this computer gets is playing Solitaire (no joke); and yet, he's faced more infections than I have ever had in a decade of file sharing, and upwards of five years with no security software.
I understand that the end-user is the most responsible party in all scenarios, but I have to say that I'm not impressed with the overall results I've seen from people running Avast. While the example of my dad is a bit extreme, the differences between him and other computers I've had to fix are not as large as you might expect. To me, it seems like the ones running Avast have the most problems. This could be a correlation with zero actual causation; however, I have had personal problems with Avast during the few months I tried running it... I'm hoping it's more coincidence, but I'm genuinely worried for the users of Avast.
I'm throwing that information out there so people have it; everyone can take it in whatever light they want. In this particular instance, I think it's worth mentioning. I've been fixing computers for friends/family for quite a while now, and have even done some paid work via word of mouth (with no one being dissatisfied), and I'm never impressed with the results of the most common security wares; Avast is the major one, followed by AVG. Even the paid Norton people have some serious problems that need addressing. Every person I've had to fix a computer for has been running a common security software, and usually sticks to the standard email/facebook/google search activities (when they don't hide their porn/dating/hookup activities from me, they obviously aren't trying to hide much before I get my hands on it). Many of these computers are also work laptops, too. Somehow, the most common security wares just don't cut the mustard, in terms of keeping your PC clean. They might prevent viruses, and major security threats, but holy crap does a lot of stuff fly under the radar; ranging from malware to silently installed programs that slow down the computer. Avast, AVG, and Norton, have never ceased to amaze me at what slips by them.