Effect of CTF on WLAN NAS access speeds

I have had an R7000 for a year or so now, happily running Tomato with a vpn connection.  I live outside the country, so we need VPN 24/7.  Internet is expensive here, so I can only afford a10Mbs connection, which Tomato and the VPN handle just fine.

 

Recently, I tried to set up a NAS drive on the network, plugged into the 3.0 USB port of the router.  It’s just a little portable drive, nothing fast, and it reads/writes about 800 Mbps when connected directly to my laptop.  When I connected it to the router's 3.0 port, though, I am only getting about a 170 Mbps read and a 60 Mbps write.  I did some research (because I am not very skilled or literate in IT stuff) and found out that fast NAT is not available for my version of Tomato but that CTF was.  When I enable CTF in the Tomato GUI, I get the same speeds, perhaps a tad faster.  This confirms what has been stated in this forum; CTF has little to no effect on WLAN NAS access speeds.

 

So, my question is simply, what if anything can I do to increase my speeds for accessing my NAS drive wirelessly through Tomato?  Are these speeds the highest I can hope for?  As stated, I need to continue running Tomato for the VPN, so I can’t revert to the Netgear OS.  BTW, all this was done on the 5 GHz connection.