It uses UDP jumbo frames with compression, I'm not going to get...

It uses UDP jumbo frames with compression, I'm not going to get more specific than that because it will cross into IP infringement territory and I'm not interested in dealing with that.

If you know what the above actually means then you'll understand that TCP packets being compressed and transmitted over UDP is not ideal for large files however works well for smaller ones. Sites like speedtest.net transmit differently so they work better on mobile devices by splitting up the file, making compression quicker and easier to transmit over UDP because the initial packet size is smaller.

The compression is the reason for different performance on different devices. Newer devices have ARMv8 architecture which has encryption instructions built into most SOCs.

Not interested in making an app, my work-life balance is pretty good at the moment and I don't plan on changing that.