You wouldn't like to take the trouble to read the whole thread, would you? The answer isn't "no", nor is it "yes". The answer is it makes you slower, but not necessarily slow.The answer is 'no'.
It is more to do with fitness.
Whilst being heavier is a disadvantage going up hill, it is a distinct advantage going down.
Also, once your mass is moving, it is working for you; sort of like a flywheel does for rotary propulsion.
Oh, and the maths of going up and then down hills makes any claim of one compensating for the other a load of nonsense. If it ordinarily takes 2 hours to climb a hill, and half an hour to go down (Mont Ventoux, if you like), and being overweight makes you 10% slower going up, and 10% faster going down...........well............work it out for yourself. You'd be 9 minutes behind your not-fat alter ego at the bottom.