Using HTTP/S to encrypt the traffic between the Web browser and the Web server provides very good security for the data, but it also introduces performance degradation. In general, for each request that is made to the Web server, the Web browser and Web server must go through at least 8 and up to 13 handshake messages before the actual data is sent. Also, one of these handshake messages needs the Web browser to generate a long random number, which is a slow process.
Because a Web page is usually made up of multiple requests, using HTTP/S as the protocol slows down the Web page being displayed. For example, a Web page with 15 images on it will mean 16 individual requests to be made to the Web server.
HTTP/1.1 has features that should allow one connection with multiple requests to work, but the implementation into the Web servers and the Web browsers has not been completed. There are hardware SSL accelerators on the market that will alleviate most of the performance issues on the Web server side when using SSL.
