IIS6 supports HTTP compression of the responses sent back to user's browsers but it needs to be enabled for this to be effective. Completely enabling compression is not obvious and if you think you are enabling compression by simply checking the "Enable" checkboxes on the Internet Service Manager (ISM) you probably are in for a surprise.
These are the steps I used to get compression working and verify that what I did actually worked. I used the steps described in this DotNetJunkies link as my starting point. To verify that compression is working as expected on your server, use PipeBoost.com to request the page or the file and review its response (compressed or not). Another way is to use Fiddler to look at the HTTP headers sent to and returned from the web server.
You will have to edit the IIS metabase to add the file types to the existing list for compression. To allow you to edit the metabase while IIS is running, you have to enable editing of the Metabase in the ISM. Right-click the Server->Properties->Enable Direct Metabase Edit. Check the box and click OK.
Now right-click Web Sites->Properties->Services tab.
This shows I have chosen to compress both dynamic (application) and static files. I left the temp directory (used for caching compressed versions of the static files) as the default. I also limited my cache size. Change these if desired. There is some overhead in checking to compress dynamic files since each web page request will require some processing time to save in bandwidth.
By default, the important file types are not compressed (.aspx, .css, .js, etc) so we will add these types with the metabase edit shortly.
Next, select the Web Service Extensions and add a new extension to identify the gzip.dll. By convention, I called this Http Compression and the location of the dll is in your Windows folder \System32\inetsrv\gzip.dll as shown below. Set the checkbox to allow and click OK.
Now we can edit the Metabase. Navigate to the same inetsrv folder from above. For safety, make a copy of the MetaBase.xml file. Edit with your favorite editor. Find the section for gzip (not the one for deflate) and add the lines highlighted:
The lines added have tabs followed by the types on separate lines. Save the file.
The final step is to do an IISRESET. This forces IIS to re-load the edited Metabase.xml and enable the compression.
You can now go to the PipeBoost.com site and enter your URL to verify that it displays that the page Document Status is Compressed. You can individually try an aspx page and a css or js page to make sure both dynamic and static pages are compressed. You can also start up Fiddler and browse to your page and verify the components are compressed. The browser must tell the server that it can handle compression (most current popular browsers) with a header
If you don't see the Content-Encoding: gzip response header for the file, compression is not correctly enabled.
You can also use Fiddler to see what the size of the request would be both compressed or not by changing the radion button: