<p>Is <em>CompileAssemblyFromDom</em> faster than <em>CompileAssemblyFromSource</em>?</p>

<p>It <em>should</em> be as it presumably bypasses the compiler front-end.</p>

I've tried finding the ultimate compiler call earlier and I gave up. There's quite a number of layers of interfaces and virtual classes for my patience.
I don't think the source reader part of the compiler ends up with a DOM tree, but intuitively I would agree with you. The work necessary to transform the DOM to IL should be much less than reading C# source code.