I ran OpenSpeedTest from my PC to my Raspberry Pi 4B, both connected via LAN to my WiFi router. The left screenshot shows the speedtest via local_ip:3000, and I'm getting the expected 1 Gbps up/down.
The right screenshot shows the speedtest via https://speed.mydomain.com/. I'm confident that the connection from my PC to my home server is routed internally and not through the internet because my lowest ping to the nearest Speedtest server (my own ISP) on speedtest.net is 6ms, and my internet speed is 100 Mbps up/down. So the traffic must be routing internally.
Is there typically such a massive difference between using http://local_ip:3000 and https://speed.mydomain.com/?
Additional context: The speedtest server is running via Docker Compose. I'm using Nginx (native, not Docker) to access these services from outside my network.
So you have local DNS set up?
If you ping (or dig) speed.mydomain.local, does it resolve the same address as local_ip?
Considering you are accessing local_ip:3000 and the domain on port 443, there is clearly a firewall somewhere redirecting packets or a reverse proxy on the domain but not on local_ip:3000
Follow the port chain, forwarding, proxying etc. One of those will be bottlenecking. Then figure out why
Edit:
Just because your ISP speed is 100mbps and you are seeing 500mbps, doesn't mean the connection isn't hairpinning through your router via it's public IP (as in, the traffic never leaves your router, but still goes through it)
yeah, traceroute might hint at that, if this is what is going on.