Also, how do you debug code that's hung or taking too long? It might be useful to get a list of all the jobs (incomplete Futures) that are currently running, much like running 'ps'.
In terms of debugging, there's not infrastructure currently, but the kind of thing you're talking about should be easy to add!
The challenge of these state machine codegen abstractions is not performance. It's making source-facing debuggers do the right thing and handle errors in a way that doesn't break the illusion that the code running is the code in source.
If you can match the degree of support C# has for error handling, you'll be in an amazing place.
Like other kinds of debugging info, you might also want a way to strip out the job names in a production build.