Er. No? That's the whole point: it does not mean that at all. Not only are there far more variables to consider than the CPU, it's not even true of the CPU, because your app will be more complex than that, and your OS runs dozens to hundreds of processes next to it.
We'll be generous and assume that you mean twice-as-fast single-core results (because your assertion is even sillier for multi-core performance). That will yield nowhere near twice the overall perceived performance of the app.
This implies a separation (e.g. into different threads that never need to sync with each other) that doesn't actually take place in practice.
And no, most GUI apps never have some isolated algorithm as their "critical path". That was kind of true in the 1990s, but these days, you waste far more time either fighting for that single core, synchronizing between threads (if you bother writing it like that), or, most crucially, waiting for I/O.
Yes, which is called a synthetic benchmark.