It will finish the maths - then process the key press. It does mean that the sketch can become unresponsive to the mouse and keyboard if you are executing lengthy functions.

If you want to process the key event while the maths is going on then the maths must be in a seperate thread.

what is a seperate thread?

During draw(), Processing queues every event that may happen at that moment.
Let's say a user hit the keyboard once and clicked the mouse twice while Processing was busy within draw().

As soon a frame is rendered, Processing turns its execution attention to those input callback functions,
as for many times as they were queued!

It's very different from those .Net Framework based languages, which automatically creates a separate concurrent thread for each triggered event ASAP. And they all run in parallel to each other!

In Java/Processing, if you want to have a separate thread, you have to explicitly call 1 yourself.
It's not automatically created for you!

Take a peek at this post link below to have an idea how to make 1.
Although for real event listeners, they are more elaborate than that to set up:
http://forum.processing.org/topic/threading-background-threads-inherently-slower-than-animation-thread