Flying fast is a numbers game- never perfect but we can always improve by bettering our predictions and playing the odds more to our advantage. There is thus no place in this for speculation and hoping for the best. It is about deciding on plans A, B and C, and ultimately our ability to fly reliably fast and thus far will be dependent on the quality and statistical reliability of those decisions. Grasshopper Related to all of the above- emotion, distraction, and failure to change gears. Thermals cycle, and sometimes take over 15 minutes to come through a trigger.
Chances are, it is the wide, amorphous lift that signals the proximity of a core.
Look for the core, while always staying within reach of you safety net of zero sink. 15 minutes feels like an eternity, but a core will almost invariably come through and it will be no use to you if you are on the ground. It might be ridge lift, but if you are low, anything that's not down is great. Far too many pilots make desperate speculative decisions hoping to carry on and blunder across a miracle climb somewhere else. If you are low and its weak, your best bet is patience with a source that is sustaining you.