Linux to Windows slow network performance

[JeffPalmer]

I'm using a CentOS 7 system as a firewall/router. If I log into the system, I get around 470Mb speed from speedtest.net on the NIC facing the internet. On the LAN side, iPerf3 speeds are gigabit between my CentOS 6 and CentOS 7 boxes. iPerf3 Windows to Windows systems are also gigabit speed. iPerf3 Windows to my QNAP RAID (Linux based) box again show gigabit speeds. But when testing iPerf3 between a Windows system and CentOS (6 or 7), the speed is 100Mb. This is also reflected in file transfers and the Windows boxes getting only 97Mb speeds from speedtest.net (which makes sense if the CentOS system is only speaking to the Windows systems at 100Mb). If I bypass the CentOS box and connect the Windows box directly to the internet, I get 470Mb speeds from speedtest.net (the same as CentOS does).

It's not the switch as CentOS to CentOS is gigabit, as well as, Windows to Windows, and Windows to QNAP (it's just a dumb DLink gigabit switch). It's not the NIC in the two CentOS boxes as they communicate full speed with each other.

EthTool reports 1000Mb links. MTU is 1500 on all systems and full duplex. I also do not show any packet errors. It's almost as if the connection between the Windows boxes and CentOS is being throttled (I checked the NIC settings in the QNAP and they appear to be exactly matched to the CentOS boxes, yet I get gigabit speeds from Windows to the QNAP). There is no firewall on the LAN side.

I am at a loss to find the issue. Especially since the Linux on the QNAP RAID can transfer data at full speed to the Windows systems. There is SOMETHING on the CentOS side that is throttling Windows systems.

Please help.

Update:
I'm was copying files from this server to the QNAP NAS and found I'm getting less than 300Mb (about 32MB) transfer rate. About twice as fast as transfers to/from Windows systems, but still far slower than file transfers from Windows to QNAP. So why I get gigabit speeds only between the 2 Centos boxes is a mystery.