I have a video file on my Android device (One Plus 7 Pro). I use the AirDroid application to download this file to my desktop computer through a web browser or an application on the computer. All this happens locally, I can cut the Internet connection in the middle of the transfer and it will not prevent the transfer.
It seems to me that this transfer is slower than you would expect. The fastest speed I have achieved is 6 megabytes per second, which I only achieve when I have an access point with my phone and connect the desktop to it.
If I connect the phone and the desktop to a WiFi router, I will only get about half that speed.
Both devices are capable of loading and unloading data much faster than this. If I connect them to an Internet connection fast enough through WiFi, they can upload and download files to and from the Internet at several times the speeds mentioned above.
If your hardware is capable of doing it when it is transmitted to / from the Internet, why can not they do the same locally? I suppose that once the bottleneck of the Internet connection was eliminated, the speed would be much faster.
I've noticed that this speed limitation also happened with other applications that did similar things, like ShareIt. I have also noticed that there is a multitude of Android devices of various brands and computers.