Normally RCX code is actually interpreted with a firmware system into code to run on the Hitachi HC-8 microcontroller - see RCX Specifications. This however can be slow, and even NQC relies on this. There is an accelerated firmware from Dick Swan to help this - but it is still not as fast as running real native code.
BrickOS actually allows you to compile binaries to run on the Lego RCX so you get a level of speed and control not available from the other systems.
This may come at the cost of increased complexity, but if you are already a seasoned C/C++ coder, you may find this more comfortable than trying to learn one of the other syntaxes or programming systems for the RCX.
If you are a Java person, you should probably consider LeJOS instead.
- http://brickos.sourceforge.net/ - BrickOS home