Eheheh, what a mess, i understand you.
I never try to do this, transfer a file or things like this, but with spacedesk its impossible to do many things when you are in wireless mode.
Your only way for use this app is connect your smartphone in tethering mode , via cable not wireless!
In this case everything is fine, without frame drop or stuttering,in other words becomes a monitor for working or gaming and this is really useful!
This is not your case because tethering mode is a connection via smartphone, not between two PCs.
In any case spacedesk is slow like other similar programs, but in tethering, nothing to say.