Another forum member runs as public “Bandwidth Test” server, see:
http://forum.mikrotik.com/t/public-mikrotik-bandwidth-test-server-s-now-shutdown-as-of-april-1st-2025/94863/1
It be curious if that also shows ~400Mb/s. FWIW the public test server runs Proxmox too…
Another question are you using VirtIO as the network interface in Proxmox config for CHR? Or some emulated network adapter? The later likely be much slower…