D-Link DIR-825 :: How To Maximize Wireless Transfer Speed On A Network

My Home/Small Business network now utilizes several D-Link products.  It is comprised of a DLink 205 cable modem (old but reliable and perfectly adequate for the speed I get from my ISP); 2 - DIR-825 Dual Band WiFi Routers (Primary - Rev B1; Secondary - Rev A1); a DLink 5-port Unmanaged Smart Switch (GO series); an HP Proliant N54L Microserver with 6 internal and 2 external HDD's (total of 19TB); 4-5 workstations (HP Laptops); 2 - Brother Laser Printers/Scanner/Copiers; a Toshiba Thrive Android Tablet; an iPad Tablet; Android Smartphones; iPhones; and a large screen HDTV.  The Modem, WiFi Routers, Switch, and Server are connected together with CAT6E ethernet cable.  The printers are USB 2.0 to the server.  The external HDD's are USB 3.0 to the Server.   

When I measure transfer speeds of Large files (single 30GB iso movie files) between HDD's internal to the Server and USB 3.0 external <=> internal, I can achieve transfer rates of 140MB/sec - 150MB/sec.  This is the limitation of 7200rpm HDD's.   When I measure the transfer rate from the Server to a Work Station by CAT6E ethernet cable (25ft length) connected to the DLink 5-port gigabit switch, I measure transfer speeds of around 65MB/sec - 70MB/sec with a peak transfer rate of around 95MB/sec.  This is ok but not as fast as other people seem to get.  I would be hope to see rates in excess of 100MB/sec.   Theoretical maximum for 1GB being 125MB/sec but the specs on the ports say they should operate up to 2GB (250MB/sec).

Wireless transfer rates are where I really am having trouble.  Transferring the same 30GB iso movie file over wireless between two Work Stations or between a Work Station and the Server gives a maximum transfer speed of 7.2MB/sec and an average transfer speed of around 4.5MB/sec.   If I am streaming multiple streams and trying to transfer a iso file, the transfer speed drops down to as low as 350KB/sec.  This is too slow.

My network is setup to do Bare Metal Backups of the Work Stations.   This can entail transferring up to 100GB of mixed files to the back-up drives on the Server for each Work Station.   At 4.5MB/sec it takes just under 7 hours to back up the Work Station.   If multiple Work Stations are backed up, it takes longer.  Too Long to be realistic.   

How to increase the transfer speeds on the wireless network?  I have seen other people post wireless transfer speeds for their networks in excess of 13MB/sec.   If I could get 7MB/sec or above when backing up all the Work Stations simultaneously. That would be doable.