We are in the process of putting together a config setting database which will describe all the settings in more detail. No ETA yet on when that will be available.
If the config file says to compress a file, booster will compress it and compare it's size to the original uncompressed. If the uncompressed size is the same, Booster will send the file uncompressed. The compression log shows everything just so you can see what is and what isn't being compressed. In some instances compressing a small gif file will actually make it bigger!
WebServer time is the time it takes for the web server to process the requests, from the time booster sends it to the time booster gets a reply from the webserver. Total time is WebServerTime plus the time it takes for booster to do its thing.