Avast! has a network scanning component in there that real-time scans network traffic. It's quite CPU heavy so on slower processors, speeds do suffer greatly. A 2.2Ghz Pentium Dual Core for example, goes 100% CPU with one core Windows Interrupts, another core Avast! scanning when downloading at close to 8MB/s. Disabling Avast! brings usage down to 30% CPU downloading at 100Mbps on the same CPU.
You can shut off the scanning component (Network and Web shield) or disable some of the more resource intensive scanning features of it.
I have a quad core i3750 and an SSD so it was not a problem of CPU useage at all. It was totally just sapping my bandwidth.
I wouldn't be surprised either way. It tends to sap the bandwidth in recent months on my OC'd i7-920 box whenever I have it plugged into Datacenter Internet or FiOS. The older versions of Avast! I know were much lighter overall. Seems to have either taken on bloat or needs some optimizations.