Your sitting in the salon and remember seeing these old fashion hood dryers and read somewhere that it speeds things up? Well why don’t you have one?
When you add heat your cuticle becomes swollen, which will cause it to be weakened. When using a bleach ( which is where most use heat) it lightens the hair faster by shrinking the gold molecules but once the swelling subsides the level of gold returns hence two or three days later you think its brassy.
When you are colouring it can not be made to happen faster without consequences so it’s best to allow the right amount of time.