#172 Linksys WRT1900AC modded slower than WNDR3800

I modded my WRT900ac v1 router with two Atheros AR5BHB116 wlan modules, however, I find that the download speeds I achieve on it are somewhat slow compared to on my Netgear WNDR3800. I've tried a number of different modules with the ath9k driver just to see if it was an issue with these particular modules, but they all give me poorer download speeds. Using speedtest.net, I consistently achieve speeds of ~119Mbps using the WNDR3800, while the modded WRT1900ac gives me speeds of ~80Mbps. Is this a known issue that others have had? Or might this be an issue particular to my router. Edit: to add, the ethernet speeds on the WRT1900ac are equivalent to those achievable on the WNDR3800, so it's something to do with wifi.