One way to improve performance is by doing IIS Compression. IIS can be compressed Statically or Dynamically. IIS 6.0 supports this.
Lets see what is Static Compression is:
Static compression To compress static assets such as .js files, .html files, and others this method is used. Static compression is a very simple process with little or no impact on server load. In this approach, the files are compressed the first time they are accessed, and are kept in a temporary folder and served to the user from the temporary directory from then on. In the case of Office SharePoint Server, all files in _layout directories (for example, .js files and .html files) are compressed. All other files, including files that are static in nature, are not treated as static if they are stored inside Office SharePoint Server. Mostly the large core.js file is compressed, and its size is reduced from about 250 KB to 57 KB. In addition, other system .js files such as init.js and init55up.js are compressed, which solves our biggest problem involving page size.