I was wondering, does encryption used over the Internet in a web browser noticeably slow down the speed of data transfer? I keep reading about stuff like HTTPS, but I always wonder if there's a speed disadvantage in securing your data over HTTPS, encrypted instant messages, or even an encrypted LAN VNC connection.
It is a question of rates. If your CPU can encrypt data at 30MB/s, but you can only transfer data at 10MB/s, then encrypted communication will only be slow relative to unencrypted communication if there is something else hogging the CPU and bringing the encryption rate below 10MB/s.
If there is something else restricting the transfer rate (say the time it takes to read from disk the data being encrypted and then transferred), then that also could be a limiting factor, but would not be due to encryption.
Other than that, there is only latency to worry about, and I think most implementations are reasonably well tuned to reduce the impact of encryption on latency.