The avast team have said that with connection/download speeds of up to 100Mbps the web shield should be able to cope without slowing the download as such. So there has to be something else in the mix.

How long have you had avast ?
What were the connection speeds like on the old 32Mbps subscription ?

The only other thing that I can think of that may be an issue is if the system spec CPU and RAM might be the next bottleneck ?