An uncompressed video stream at 240p, 24 frames per second is 60 mbps, not really something an IoT device can handle. And if the video is compressed, decompression is going to be significantly more expensive than AES - adding encryption is not a meaningful computational overhead.
>1 operation per second would refer to cryptographic signatures. If you are doing Chacha, the speeds are more like 1 mbps. AES is probably closer to 400 kbps.
It sounds to me like you, sir or madame, have not worked with truly tiny MCUs. :-)
But yes, there are inexpensive MCUs where you can do quite a bit of crypto in software at decent speeds.
Modern encrypted streaming uses pre-existing compressed video where the packets are encrypted on the way to you by the streaming server. It would have to uniquely encrypt the data being sent to every single user hitting that server. So it's not just a one and done type of thing. It is every bit of data for every user. So that scales to a lot of CPU on the server side to do the encryption. Yes, on the receiving side while your device is only dealing with the one single stream, more CPU cycles will be spent decompressing the video compared to decrypting. But again, that's only have of the encrypt/decrypt cycle
Eg a small edge gateway could be doing the VPN, while the end device is decoding the video.