Larger files take longer to download.
Compression techniques allow you to compress/transfer/decompress faster than just sending a file in its raw state.
Media files are usually compressed already but gzip compression can be enabled for files that contain just text.
Fiddler will show if any compression has been used.
Make sure compression is enabled at the server for all suitable file types.
Enabling HTTP compression
Comments and white space should be removed from all text files - use proper source control, not comments
to maintain a history. Note that compression is case-sensitive so keep ID and class names consistent.