Having set up the CLI to be perfectly asynchronous and parallelized, it occurs to me that there are most definitely systems that will have difficulty with this, or scenarios in which sequential execution may be preferred. I'm thinking particularly of systems with older Intel processors.

In light of this, I think it's perfectly reasonable to create a single-threaded mode for the CLI to make execution less straining on these systems (though it will be considerably slower). This won't affect performance of the multi-threaded mode in any way, which will remain the default. I suggest specifying single threaded mode with --sequential.