At risk of being repetitive, i’m sure that the issue of slow WAN to LAN downloads is a software problem and not a hardware related one.
My config is an FTTH PPPoE ISP provider with 200Mbs down and 60 upload speeds, and a CAT6 wired connection between ONT / router (ASUS 66U; N56U or Cisco RV180) / computer.
The WAN connection between ONT and ASUS routers is made normally by one VLAN (ID=10), so i configure the ASUS as IPTV with ISP Profile Manual, Internet VID10, PRIO 0, and LAN port 3,4 as unassigned ones, as i have not an IPTV subscription, only Internet.
With any FW after 3.0.0.4.354 for 66U, no matter how i do the FW upgrade, the downloads speeds go down from about 200 Mbps to about 140/160 Mbps, as i have reported here:
Another firmware that not fixes the restricted download bandwidth:
http://imageshack.us/photo/my-images/547/956h.png/
With 3.0.0.4.354 firmware:
http://imageshack.us/photo/my-images/513/u5br.png/
Interestingly, with HW accelerator disabled, download bandwidth goes up.
I have not updated the N56U firmware after 3.0.0.4.342, and download speeds are normal with N56U (about 200 Mbps).
With Cisco RV180, as it does not have the capability of VLAN in WAN area with PPPoE config, i have to change the ONT config to PPPoE bridge, binding VLAN10 in one of ONT ports, and the measured download speed is also about 200 Mb.
So, if with previous firmware for 66U and 56U i can obtain download speeds of 200 Mb and not with recent FW, the issue is firmware and not hardware related.
This is additionally confirmed as the RV180 with its modest hardware capability (300 MHz CPU, only 64 MB of RAM and small 16 MB flash memory), is also capable of 200 Mb downloads, even in PPPoE configuration.
About firmware 3.0.0.4.354 for AC66U there is an intriguing fact: we must do its flash with the WAN port open, otherwise the download speed is also cut down to 140/160 Mb, as i have mentioned in post #18.
In a rough supposition, i think this could be related to QoS being active if there is an operating connection (PPPoE only?) between the WAN port of 66U and ONT, this way disabling HW acceleration, but with more recent firmware, anyhow i do the flash the download speed is reduced.
Sorry by the lengthy post, but i think ASUS AC66U with its WRT derivative firmware has serious problems, at least for PPPoE connections, for anyone with download speeds higher than about 140 Mbps, as already noted by many people, and also think they should already have fixed it.