BitDefender works well for me too!
Bit defender is horribly slow and it updates always at the wrong time and slows down pc. I prefer more discretionary ones where i can set when to update it.
Yes blame the software because it runs slowly on outdated machines. BitDefender then Kaspersky -> others.
I never noticed BitDefender updating. It takes 200MB of RAM which is less than 2% of my total (unnoticeable).
I have a very good machine. I7 core laptop with 4 gb ram and still it slows the crap out of it and mostly it updates randomly so when i run a resource intensive application it starts to update to slow it down even more...
I used AVG and its pretty smooth however it didnt detect a few viruses, so i`m using Kaspersky 2015 now and it works like a charm.