Typically very little. Most current CPUs integrate some variation of the Advanced Encryption Standard instruction set (AES, AES-NI, etc), so just like H.264 or

Typically very little. Most current CPUs integrate some variation of the Advanced Encryption Standard instruction set (AES, AES-NI, etc), so just like H.264 or H.265 decoding, it can be very, very efficient with custom instructions (magnitudes of order faster / less power consuming than without). This is why most devices are equally capable of rendering http or https (you never have to 'revert' to http because a website is too slow in https... it's just not a thing). A stupid background app may consume 10x or 100x your encryption budget.