What you have to appreciate is that you have a hot Exhaust pipe less than 2" away from your leg.
When cruising slowly around town there is not enough air flow to take the heat away, it's just radiating outwards.
Once you get out on the open road and are at cruising speeds, the air flow past the pipe is sufficient to take that heat away.
As mentioned earlier, even a stock bike will run hot around town as they are set up lean for a start.