Your title is somewhat confusing firstly are you talking about the Comodo anti virus, as AVG (Grisoft) has nothing to do with Comodo as far as I'm aware
If you meant Comodo AV then you have answered your own question, slow with Comodo, not with avast.
Not to mention avast is the most configurable and flexible anti-virus I've ever used and avasts detection rate is far better. Comodo AV is also a very young and developing AV.