It depends on the encryption algorithm used. Strong algorithms are very resource intensive. The link I posted above has one paragraph:
"From the UI, the user selects to encrypt the device. The UI ensures that there is a full charge on the battery, and the AC adapter is plugged in. It does this to make sure there is enough power to finish the encryption process, because if the device runs out of power and shuts down before it has finished encrypting, file data is left in a partially encrypted state, and the device must be factory reset (and all data lost)."
In the traditional Web scenario, HTTPS encryption is a heavy process as well. It is pretty common to have the server use a hardware accelerator.