Pretty average file transfer on network - help troubleshoot

Hi I have a Linksys WRT1900ACS - device A connected at 1Gbps wired; device B gets 500Mbps when doing internet speed tests over wifi

Why would it be that when I transfer one large file between Device A - wired to device B - wifi, I can only get 20MBps (ie around 80Mbps) when there is nothing else going on in the network?

Is there a setting to tweak or do I just accept it?