With no argument this returns the current process's priority. 0 is the maximum priority and 19 is the minimum. Given a non-negative integer, it will increase the priority value by that amount.
If you always want your Lens processes to run niced, put a call to it in your .lensrc file. However, this is no longer really necessary since Lens will automatically renice itself to 10 if it is actively chewing cycles (not idle) for 10 minutes. This lets short interactive process be very responsive while long jobs will be nice.
To be nice:
lens> nice 10 10
To be really swell:
lens> nice 20 19