I was working with a client recently that was experiencing poor performance on their SharePoint environment. They were about to go live, but the performance testing was coming back with terrible results, even with only a small number of users. Initially we weren't sure where the problem was, but we were able to quickly isolate the issue down to network performance.
We noticed that certain users were able to upload quickly, but for other users the file upload process crawled. We ran up Performance Monitor on our front end SharePoint server to see what was happening. Adding the Total Bytes/sec monitor in the Network category gave us some useful information. For the users experiencing the good performance, our monitor would spike and then drop back to zero (the blue line below). For those experiencing bad performance the network utilisation would stay low. Something was throttling the network bandwidth for these users
To cut a long story short, the client finally discovered that the issue was to do with a default setting in Internet Explorer 6.0 and earlier. This limits the data rate that the browser sends information. This becomes particularly noticeable with larger files (e.g. greater than 2 MB).
To resolve the issue you need to make an update to the registry on the user's computer. You can read more about this here