For scanning speed,
For me, besides Clam, Kaspersky (and its clones e.g. F-Secure), BitDefender is one of the slowest AV I've ever seen, I used to test its latest version on my personal laptop, desktop and several machine on corporate network but all gives the same results, slow on-demand scanning speed and slow down entire system in real time, so I don't care about BitDefender is implemented with super-advanced heuristics or something else if it slows my machine down.
Although avast! is not that fast as NOD32 but for me avast! is not as soooooo slow as Clam, Kaspersky, BitDefender.