VNC can have good performance over gig LAN using raw encoding. The issues occur when you add compression (hextile, tight, etc.) which does a good job cutting bandwidth but adds a ton of latency, which is the main reason your sessions feels slow. RDP does a great job of keeping the latency down.

Note that I have never had a good experience using noVNC (the web-based solution used by this service) even over fast LAN - I think it's because of the additional overhead of the web browser.

I personally use SPICE for VDI which performs decently over LAN, but is not as good as RDP over WAN with constrained bandwidth.