I was using SLIRP a few years ago to tunnel traffic through a cheap OpenVZ VPS, (for downloading Linux ISOs). I had to manually patch the TCP window size and recompile the binaries to get decent speed as the original code doesn't support TCP Window Size Scaling. I tried to upstream the patch to Debian but the maintainer wouldn't reply. It worked well enough but now I use wireguard.

Modified SLIRP code is also found in VirtualBox, Qemu, UML and other virtualization software, for sharing the host connection in NAT mode.