In my experience, SPICE is awesome on a fast local network. I even run it at 4k with a remote VM. I experience more frame drops than straight up lag - but things like video and audio are surprisingly good.
I've been searching for a good (free) virtual desktop/workstation solution but Proxmox as a host + SPICE so far as the best I have found. I'd love to also find something free that supports virtualized or pass-through GPU. I agree that SPICE is not perfect, but it's the only protocol that feels like I am actually using the remote machine as a local desktop unlike VNC or RDP.