Improve performance and stability

sprint#9 :fast_forward:

[ ] Start by profiling the entire application (engine) to see where the biggest problems are (@josteinaj, @bertfrees)
- We're going to try to work with YourKit. I've done some preparation by writing some "probes": see https://github.com/daisy/pipeline-build-utils/pull/19.
- Jostein has already done some profiling before on another script. One conclusion was that Calabash might be a big problem, and that is exactly what I've been suspecting as well.