http://tirania.org/blog/archive/2009/Jan-20-1.html

The new code generation engine is based on a linear intermediate representation as opposed to the tree-based intermediate representation that we had used up to Mono 2.0.
Mono's new engine generates much better code than the version found in Mono 2.0.

Speed: The engine will mostly benefit computationally intensive code, usually between 10% and 30% performance increase, with some cases going up as high as being 50% faster.

Code size: the new engine generates slimmer code, typically 12% to 20% smaller code generated