I always felt that agile processes were a means to give non-answers to performance measurement in order to allay the consequences of that measurement and allow developers to get on with the business of making software.
How big is the task; 10 story points
How long will that take; depends on our velocity
Can it be done by this date; we're not allowed to plan that far ahead. You can put it in the 'icebox'. We might get to it eventually.