Heat conduction rates ultimately limit the extrusion speed, but there are lots of variable to consider. I'll try an dump what I know to be factors. Dig in.
1. Peek, used as an insulator in the cold (extruder) to hot (melt pot) interface melts ~240°C. This sets an upper limit of around 230°C for the melt pot.
2. It has been found by many that a short melt pot is better at avoiding jams, and less mass means more speed, but the heat is pulled out of the smaller mass of brass very quickly, so the heat transfer from the resistor needs to be very efficient. Having a very thin wall between the resistor, and the melt pot helps improve this efficiency.
3. Using bowden extruders reduces overall mass, but has other problems with reversing. Don't know how well these issues are currently being addressed.
4. Filament chemistry varies greatly by supplier/batch/color/day of the week, etc., so you might get great results one day, and nothing the next. This is true for all types of printers, not just deltas.