Thanks for that response. I have tried multiple browsers and different PCs, still the same. It seems to work ok (ish) for a while then go really slow. Running a top command from the CLI doesn't show anything taking up significant amounts of resources.
Unfortunately my project is happening right now, not at an unknown point in the future, so if I lose my patience with it again today it will be going back in the box and I'll use a Mikrotik instead. I didn't buy the router in the future, I bought it this week. If something isn't ready to release to the market then you don't release it or you advertise it as "with a faulty webUI".