slow temp rises
You have fused a piece of glass that is now 6 or maybe even 9 mm thick, so you have to go slow for two reasons at least. 1. you have to give the glass the opportunity to get hot all the way through. 2. fast temperature rises lead to breakages either on the way up or during the slump.
So a rise of ca. 100 C or 150 C is as fast as you want to go. When using slow temperature rises you often can achieve the work at a lower temperature, and with a longer soak are certain to get the work done. Unless the glass is all of one colour/sheet all across itself, fast heating will lead to the darker and softer (not the same) becoming plastic before the rest of the sheet and even before the bottom is soft enough to begin to slump. That is when the weight of the glass above can split the bottom without completely breaking the piece.