Yo won't loose any speed as long as you train right.
You will if you force bulk onto your self.
How old are you? If you're like 16 I bet you end up putting on 2 stone by the time you are 18

Sprints and Plyos will increase your speed and if you are eating right probably put mass onto you.

Safe if you work the bag properly, if you're just doing all of these pitter patter punches constantly you'll just gain muscular endurance and stamina... but if you train properly you'll gain that plus speed-Strength (or 'power )