NCrunch will rebuild all dependencies of a project if they have the 'Copy referenced assemblies to workspace' setting enabled, or if you make a change to the project that impacts a public interface (i.e. you change the signature of a public method).

In all other situations, NCrunch will do exactly as you've described - it will re-use the previously built assemblies for these projects and simply go straight to running the tests.

Are you making use of the 'Copy referenced assemblies to workspace' setting? If so, do you have any opportunities to turn it back off?

Unfortunately NCrunch does need to rebuild dependencies when a public interface changes. This is to avoid obscure problems such as 'Method not found' exceptions that can otherwise be thrown during test execution.

Note that NCrunch won't rebuild projects that do not have a direct or indirect dependency on the changed project.

Something you may be interested in is the recently introduced build priority configuration setting. This allows you to control the priority of build tasks added to the queue. If you have multiple test projects running tests at different hierarchical levels of your solution and the engine is tying up resources rebuilding when it could potentially be executing tests (for example, if the lower level projects have already been rebuilt), you may wish to reduce the priority NCrunch assigns to build tasks so that more resources are put towards testing.