Slow file transfer

I have an issue with slow file transfer (windows 10). I am almost sure it has nothing to do with the network settings.
I have one application and:
1. If launched from local drive works perfect
2. If launched from mapped drive (same folder on the same hdd) works very slow
3. If launched on another computer in network very slow as well
It seems to me the issue is mapping the drive itself. Even with no access to the network result is the same:
disk C:\Folder - full speed,
disk F:\ (mapped disk C:\Folder) – no speed at all.
I’ve checked all solutions found , and still no result.
Any ideas ???

IMO it has nothing to do with hardware. Physical location of files is the same (disk C folder) - mounting virtual drive slow down the transfer very very significantly. Additionally it has to be done that way because another 4 computers use this software. It is standard Windows 10 workgroup.

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Please see the screenshots. It starts with more or less good speed, than transfer drops for less than 1 Mb/s. Physically the same hdd - Samsung 500 GB ssd, so quite fast.

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Anybody????
In meantime I've checked transfer on my son computer. The same situation, after twenty seconds transfer drops from 200 mb to 1 mb and stay so low.
It seems to me "it's not a bug it's a feature".
Any ideas i.e. external linux serwer or something else?

Any ideas concerning slow file transfer? Till today I didn't found the solution, even Microsoft didn't answer my question.