Transparent proxies can simply pass the encrypted request on to the destination, performing absolutely no processing. However, because the request passes through the proxy encrypted, it cannot cache any of the pages, reducing any potential performance benefit.
Application layer proxies can unencrypt the request from the client, and send an encrypted request on to the destination server. This would provide for caching to take place, but adds overhead for the added processing necessary to encrypt and decrypt. This processing is usually very, very minor. If it were causing a problem, you would be able to diagnose it by watching the Processor Utilization performance counter within the Processor performance object in the Performance administrative snap-in on the proxy server.
This was first published in May 2002