If I gain muscle mass, do I lose speed?
Believe me I could write an essay on this very topic but as I don't wish to bore you to death - the short answer in general is probably yes. Why........? As far as GRAVITY is concerned there is no difference between muscle mass & fat mass. Or for that matter between water mass or bone mass ?
If you have ever been skiing & are reasonably observant you will have noticed that in general a fat skier gets to the bottom of the slope faster than a slim skier !! Why ? The fat skier obviously weighs more than a slim skier. Similarly a muscular skier will get to the bottom of the slope faster than a slim skier. Why? The muscular skier obviously weighs more than the slim skier. Now compare the fat skier with the muscular skier- you will find that they will get to the bottom of the slope at roughly the same time as long as they weigh the same. In fact the muscular skier will probably just about win the race because he is more compact & has less surface area than the fat skier & so has less friction from wind resistance. But ultimately there is a delicate balance between gravity & wind resistance ( friction).
Now getting back to boxing- consider the following image: