It’s a server with integrated UPS and KVM console.

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)

You could put lining paper over the cork, so you have a paintable surface.
My flat has plaster render over brick for internal walls (external walls are lath & plaster).
My hallway has a styrofoam backed wallpaper. It gets damaged SO easily, and I haven’t found a decent way to repair it. I presume at some point, I just need to repaper it.
I think cork is a bit hardier than styrofoam, but just a heads up for high traffic areas