Thank you for signing up, vj! Hash rates indeed drop when you navigate to other tabs, but all in all you still do it. You can try clicking the Settings button under Mining and adjust the hash rate. I can only raise it to 4 though without it clogging the browser. I usually keep it at 3 when I am not around using the computer and 2 otherwise.
They are still tweaking and testing and working things out - there have been several updates already in which they improved hash rates and I am sure that it will get even better in the future.