I know three additional possible reasons not discussed yet:
one is that antivirus checker slows down (less probable, if the laptop is fast)
the network MTU size of the sender is bigger than allowed by WLAN, so the network packages need to be repacked before sending
the WLAN encryption could slow down speed also (with a slow Laptop)
As proposed I would try another sender, and then I would search a good WLAN Analyzer.