Posting this here for other people to find. I had significant issues with package loss on my GL.iNet MT1300 router. Imagine you're doing a ping -c 50 to your first hop and 20-40% just drops out, usually in small batches. E.g. package 1,2,3 arrives 4,5 disappear, 6,7,8 arrives and so on.
It turned out my ISP is actively making some sort of decision based on the MAC address of the device!
Changing my MAC to a well known large chipset maker made all my problems disappear like snow before the sun.
I can speculate as to what exactly is happening, but I can't influence that and I doubt they'd care if I'd inform them. Either way, if you are experiencing weird package loss which you don't have when connecting e.g. a laptop directly then it might that your ISP is the culprit and not you, nor any of your OpenWRT settings.
The long story for those who are bored and want to read something:
I have a very simple cable modem (DOCSIS) which can only bridge. I used to have a regular x86 computer behind it that did firewall and so much more, with other OpenWRT devices behind that for WiFI. It's a good idea to just have a dedicated piece of hardware as the entry point for your Internet connection and not a jack of all trades machine. After all, you mess up and other_people will complain that there's no Internet (uh oh). So, I looked around and settled on the MT1300 for various reasons. Sometimes it takes a while for devices to arrive, hence I started off with a raspberry 2b I had lying out figuring it would make an easy copy paste while I figured out everything I wanted to run off of it. That device started off with weird package loss. It's using a ethernet over USB as second port, maybe it's the USB bus going idle. Nope, maybe it's the power then? Nope, maybe it's the cabling close to the adapters that's causing crosstalk? Nope. Well.. I gave up on that waited it out for the MT1300 to arrive (while my favourite streaming service hiccuped away and any voice calls became staccato).
My MT1300 arrived, copied config and almost the same problem. My ping times were better, but I still had package loss! Well, you think it must be a problem then with the coaxial cabling right? Got technicians over, nothing, clear perfect cable signal. Well then it had to be the cable modem, borrowed a test modem and wow - good connection. The test modem was a fancy router doing NAT, WIFi and so on.
I get myself a new dumb cable modem, identical to the one I though had crapped out and ... nothing. Identical problems. At that moment I had already spent quite a bit of time wasted in frustration and I was running out of ideas. I joined the OpenWRT IRC channel and one person gave me some really good ideas. For instance, double checking that the packages get sent out using tcpdump-lite (they did). Running a computer directly on the WAN port making sure that there was nothing wrong there either. When all was said and done the only real difference between a direct connection from computer to the cable modem and a connection to the MT1300 was... the MAC. I mean, I even considered things like EEE (Energy Efficient Ethernet), link layer properties and so on.
Changed the MAC and lo and behold! Part of me was flabbergasted, part of me was fuming because of all the lost time and effort on something that turned out to be a non-issue.
Either way, I would like to thank the people who helped me bounce ideas off them and who gave me some really good ideas to consider!
(edit: spelling, minor clarification)