It's never mentioned how hot the air temprature is on Mustafar (The Lava Planet) nor is it ever mentioned in any 3rd party reference (books, website) Lava usually measures at a minimum 1800-2280 F (1000-1250 C), Now if the entire planet is covered in lava, the entire planets surface would be like one giant stove or oven. Obi-Wan and Anakin could not fight as freely as they are seen doing. they would only survive for a few seconds on the planets surface, after that they would've roasted.