VNC is unfortunately inherently inefficient because it is just a framebuffer protocol, instead of RDP which passes through the graphics primitives to be rendered. The former will always involve encoding/decoding overhead at the server and client.
Sending graphics primitives turned out to be the worst way to do remote desktop. All modern solutions just use video codecs. NX (the best solution on Linux) even switched from X11 forwarding to a video codec in NX4.
VNC is inefficient because it is ancient and uses extremely inefficient methods to encode the graphics. GIF is really inefficient too but you wouldn't say that means the idea of encoding animated images as bitmaps is a bad one.
I guess “by default” means if both client and server have the required hardware support and there aren’t too many concurrent sessions.
But IIRC, by 2007, it switched completely to sending tiles just like VNC.
However, if you read the protocol descriptions, you get the wrong idea that primitives are still used.
Similarly, most X11 clients have been doing everything client side for ages, but many people still believe that peimitives dominate.
An example of a truly inefficient encoding method is ZRLE. It is a tiled zlib and run-length encoded format that can't be split up into multiple jobs because future computation depends on past computation.
They got things right with the tight encoding method of which there are two variants: zlib (lossless) and jpeg. With zlib, you can have up to 4 separate zlib streams, which means that you can utilise 4 CPU cores for encoding in parallel. The jpeg method has no such limitations.
RDP also supports plain framebuffers and that's what we use.
Anyhow, with the advent of specialised hardware for video encoding, passing framebuffers isn't necessarily such a bad option. You just have to make sure that the CPU doesn't touch the framebuffer before encoding and after decoding.