VNC and RustDesk are both remote desktop solutions, however, Rustdesk is considerably more performant than the VNC family, because the latter primarily sends framebuffer updates, while the former can use modern video codecs and temporal compression to encode screen changes much more efficiently.

VNC is one way of doing remote GUI access (well, more a family of different protocols and products with different capabilities and tradeoffs). RustDesk is just another product that does the same thing but doesn't have a specific connection to VNC as a protocol so e.g. doesn't have to handle legacy authentication modes and stream formats. In my experience most remote access solutions beat VNC for performance, for example.

Well first there was TeamViewer which was VNC with more bells and whistles then it became enshittified. Then anydesk came along and ate up teamviewer then that became enshittified, Now we have rustdesk which seems to hopefully be a bit more immune to being enshittified.

If you already use VNC this is not something for you