I have a Netgear WNDR3700v2 with OpenWRT 19.07.2 (oh, I see 19.07.3 is released) connected to a PPPoE service with 100/100 Mbps internet service.
I'm thinking about a faster service, so I wonder, if my router can handle it.
So the questions (all for IPv4):
how much can it handle over PPPoE?
what is the typical performance difference between "plain" IP NAT routing and PPPoE (also has NAT of course)
is there a noticeable difference between default settings and non-default? What are those settings that would affect performance?
My router has a more or less fresh install of OpenWRT, changes from default are added PPPoE configuration, and logging to a file in /var/log, and a few tools from the official repo installed (ip route2, less and so, nothing that runs in background).
It all depends on the new speed you're looking at, but something like 500 Mbps or 1 Gbps is definitely out of reach for an older Atheros device like this, afaik.
In real life WNDR3700v2 can do combined upload+download of some 100 Mbit/s if you use SQM or similar QoS tools consuming CPU power. PPPoE will also burden it somewhat.
(That 400 Mbit/s test from year 2011 sounds pretty farfetching and is likely only possible with pretty ideal conditions, and absolutely no traffic shaping.)
If you consider upping you speeds from 100/100, you likely need a much more powerful router.
PPPoE adds a significant performance overhead relative to plain ethernet, so any benchmarks using merely that is meaningless for PPPoE connections. Using PPPoE, my TL-WDR4300 (560 MHz AR9344) can just do 100/40 MBit/s (without SQM!), but the CPU utilization is right at the limit (not all the time, but often enough to show the bottle neck), so there wouldn't be much headroom left for faster connections. I would not expect any ath79 device to be capable of dealing with more than 150 MBit/s at best (if you don't care about latencies under load, it might go a bit further - if you want QoS/ SQM to work, you'd need to lower your expectations accordingly).