system PID 4 on my XP Pro is currently at 0% CPU and doesn't get high in file transfers, unfortunately on a stand alone system I can't test network transfers. As far I'm aware avast doesn't have any hook into system but I don't know about srv.sys as avast is not a server product I wouldn't have thought so.
If avast were in fact scanning something the avast icon would be rotating and the ashServ.exe would show cpu activity.
So using your examples:
"This pegged the CPU at 100%, 16% was Explorer.exe, the other 84% was the System process." This would mean there is no activity on ashServ.exe or any other avast process.
"Without Avast! the copy takes about 50% CPU, with 16% Explorer, and 34% System process." Now without avast there is still a high system cpu usage which I can't understand for a simple file transfer. Although I can't see why it would drop to 50% cpu use after avast was uninstalled, when previously there was no avast activity.
Baffling, that you are able to replicate this on other systems but those here that have tried haven't experienced the same problem and considering Alanrf's comments after uninstalling.
If I stop all avast on access protection the CPU utlization is steady at about 57%, when I uninstall avast - the CPU utilization is unchanged.
Perhaps those who tested the cpu utilisation could check the processes using the cpu time as the system process PID 4 seems high when I would have expected ashServ.exe to be eating some of the cpu % that doesn't appear to be the case with Saturn49's system.
Perhaps as Alan mentions Network Shield has an impact in network transfers and we don't see a process PID for that, so it might use ashServ.exe also or perhaps system