I was trying to install Fiddler2 to capture some HTTPS traffic in order to troubleshoot a slow file upload speed issue between client(windows 7) to a web server(Windows 2008 R2).
One strange thing I have noticed was the file uploading process became super fast after I have fiddler up and running. Could it because the fiddler started acting as a proxy server and using port 8888?
Why is it when I start using Fiddler2 the upload process speeds up?
Is this a WebDav request? Or a file upload through a traditional HTML form?
For WebDAV:
Most likely, this is due to a bug in the Microsoft WebDAV implementation, whereby they waste a bunch of time trying to "Automatically Detect" your proxy server over and over again via WPAD. This can be very slow in an environment without such a proxy server.
Fiddler caches the result of a proxy detection once at startup and sets itself as the system proxy, which prevents the WebDAV stack from incorrectly rerunning the WPAD algorithm over and over.
For a HTML Form:
Typically, this would mean that the client's buffer sizes were poorly chosen (IE6 had this problem) and thus the client isn't making good use of the network. Fiddler uses better buffer sizes (32k or 64k, IIRC) and hence makes better use of the network, reducing transfer times. Most modern browsers use buffer sizes chosen to optimize performance.