Thanks everyone!
UPDATE: I have connected the router provided by my ISP. It's a SMART/RG SR400ac. It's running a strange version of OpenWRT called SMART/OS (ver. 10.8.7.1). I don't like it, but it's getting better speeds. It's cli is strange and very locked down:
Definitely skinned OpenWRT:
It looks like it might have a BCM4708A0 (Arm9 800Mhz), so maybe this is the reason I'm getting better speeds?
It's just frustrating as I know that the AC68U is able to do better as I've had it on 1Gig fiber before.
I just need to break down and get a new ASUS...
NAT Acceleration has always been on.
turn off AI protection etc.
Ai Protection is not enabled and never has been.
router you are using technological is 8y old and was not design to 1Gbit - to be honest any home user Router is not design for 1Gbit - to have it working with VPN etc you would need to make miniPC wit FW OPNsense, pfSense or OpenWRT.
This particular router was used on a 1Gig fiber line for 8 months or so at one of my previous residences and it was just fine. This was the main reason I was worried about the performance I have been getting.
with you ISP speed you need min AX86U but you cannot turn on QoS as speed will drop to 300-350Mbit, the same with VPN etc.
No QoS is enabled anywhere and never has been.
In fact no 'extra' applications that are featured on this router are enabled.
Using SPDMerlin to evaluate the speed of your WAN connection can't be relied on if you have a high speed Internet connection particularly if you are using older routers with less powerful processors. Since you already know that your ISP is delivering the speed you are paying for your best test will be first test the speed across your LAN using a utility such as Iperf to test your networking hardware, then run a speed test by connecting a PC using an Ethernet cable to your router and then run several speed tests to a nearby server on your PC. Then repeat the test using SPDMerlin. If the test results running on the router are not satisfactory you can try doing a factory reset on the router and then a minimal setup and see if the results improve. If you still are not satisfied with the results time to upgrade your router.
I started testing using a laptop and a short, known-good ethernet cable. I was getting the same results as the router's CLI.
To your point about the processor, yeah it's a sad old Arm7 (800Mhz). It should not make a difference for the WAN performance, but I did overclock it to 1200Mhz and it really only made the startup quicker and the UI a tad bit snappier.
I also tried a WRT1900ac I have been using as a switch/repeater. It has nearly the same processor and is running OpenWrt.
I reset it back to router mode and I ended up getting the same results.
Jumbo Frame: disable
Mac address: empty(no mac clone)
Check the cable: should be 4 pair.
Yeah, these are the settings it's been at all along.
All my cables are tested and good.
SR400 Speed Tests: