The initial temperature of your tip cannot overcome the fact that you're limited by the total wattage heat output of the iron when you start dumping all your heat into a large thermal mass. You either need a beefier iron that produces more heat or, as suggested above, you can try pre-heating the board to add some pre-heat into whatever you're trying to desolder.
One trick that may work is that if you have a second iron, even a non-temperature controlled regular type one, you can stick it on there
also to pump more total heat into the job. You just need some dexterity (or a helper) and hold your tongue at
juuuuust the right angle.

At times like this I would probably just reach for my ancient, big, fat Weller 80 watt (looks similar to the SP80N or the SPG80L stained glass iron, but much older with a black handle, no idea what model it is, probably something like SP80, though

) or even just grab the good ol' Weller 8200 100/140W gun. In situations like this you're less likely to cause damage by using an "excessively large" iron and going in there and quickly heating the joint with the ginormous beast rather than sitting there cooking everything around it for ages while you try to heat it up with a small iron.