Get Apples attention it will add a little more pressure in a resolution, They can add a fault ticket linked to your order and not process the replacement straight away. Getting something logged is the best thing to do when the problem starts so you have a reference point. I would rather a fix just released than pack the thing up wait for collection and re-delivery of a new one.
I have used Apple computers for the last 12 or so years and always give them time to fix things as they always come through eventually. That aside do you really want to do a replacement and get the same fault? There is a lot of people who have requested returns get some feedback from them before moving forward. I would rather have a machine that freezes for the time being than non at all.