Avast Pro 8 slower than Version 4.8

Hi,
We have a number of Windows 7 (x86) machines that are not connected to any network (signatures updated regularly). Their sole job is to scan external USB 3 hard drives before they get put on to a network. It is very important that the virus scan runs as fast as possible.

Currently we use Version 4.8 of Avast. I have downloaded a trial version of Avast Pro antivirus 8 in a hope that the scanning speed is improved. However, the virus scans are twice as slow as in version 4.8.

We use the command line scanning. We have a batch file that does the following:
Starts the virus scan going and writes the start time out to a file.
Once finished, it writes out the finished time to a file and gives a summary of the number of files scanned and any viruses found.
It then plays an audible noise to notify someone that the scan has finished.

Is there something that I am missing?

Is there a 64bit version of Avast?
Or is there any way of making Avast use more memory / CPU?

I would really appreciate any help in improving the scanning speed.

Cheers,
Rob