If all file sharing apps like shareit, xender and SuperBeam uses wifi technology to transfer files, why their transfer speed differs?
How reading the file from the file-system can affect performance. Often there are various ways to read a file in a programming language.

The physical transfer that uses everything below OSI layer 6 can't be different, as those apps all use Androids default APIs to achieve that.

Then how saving the file into the receivers file-system also affects performance. There are various ways to save a file in a programming language.