I have Avira Internet Security installed on one of my systems and it always updates at full speed.
However, the update servers are different for the free version. Apparently Avira state that they can't afford the bandwidth costs to grant the free users with reliable speeds. I was thinking that because i was updating several machines a day from my one ip address that they were throttling me but it turned out that the free version update servers are often unreliable for everyone.
I have switched to downloading the definitions manually and explain to customers when they collect their machines that some days they will struggle to get updates.