One has to be careful when using an antivirus application that protects computer in real time (scans files when one opens them). Here is what happened. I am working on a project for a customer of mine. For source file control we use Visual SourceSafe over VPN. I won't discuss further whether this is an ideal combination... It actually performs good enough even though the line is relatively slow. However, one day it became dead slow. By dead slow I mean half an hour to open an project and 2+ hours to get latest version. I immediately suspected Internet provider and transfer speed over Internet. Yet, this wasn't it - it was working normally on another computer of mine.
So I start thinking what did change on my computer in last week or so. After exactly 5s of deep thinking I had a revelation - I installed an antivirus system a week or so ago. Combine it with VSS over VPN and you have an unresponsive VSS client. However, the problem is not within this particular antivirus application. The problem is in the VSS itself. VSS is just a file server, meaning that everything is done on the client - actually there is no such thing as VSS server (there is some thin layer when using VSS2005 over Internet). And knowing that VSS usually produces a myriad of small files I immediately figured it out what's going on - VSS client has to check a ton of these files and each file is checked by antivirus application in real time - thus totally killing the performance (multiplying * x access on each file over VPN).
The solution was simple - I disabled real time monitor for network files and VSS started to perform normally again.