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)
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)