Black tea water: Does boiling water first make a difference?
As you're heating the temperature of the water will rise at different rates, depending on the amount of heat, on the quantity of water, and on whether the pan is covered or not. With the tea in the pan, this would create an infusion gradient, as the temperature rose over a varying number of minutes. 

If you boil the water first, then infuse, you've fewer variables: the quantity of tea and the quantity of water. So you'll have less to adjust to get the brew perfect.

I don't see how you could brew even the finest black tea for 10-20 minutes without over-extracting it, producing a stewed, bitter flavour that dries the tongue.