Pinecil update: I was given a proper 30W PD adapter (wall to USB type C power adapter with Power Delivery that negotiates up to a higher voltage to go faster) this weekend, and my Pinecil now reaches 240 degrees C in twenty seconds.
That's at 12 volts - I suspect the USB cable is now the bottleneck, it should be hitting 20v with this adapter. But 20 seconds is probably as fast as I'll ever need or want a soldering iron to come up to temperature.