the HB has a 150 shot clip, reloading is an optional extra most of the time so the reload time is mostly irrelivant.
To put it into perspective 3-4 HB shots will drop a warrior. The Heavy bolters true flaw lies in his speed (or lack of it).
umm... your heavy bolter marine reloading should be the crucial and tense moment where the rest of the squad has to double their efforts in order to not be swarmed. For the Marine to just wait two seconds and start annihilating everyone again would kind of eliminate that opportunity for the Tyranids, along with the tension that goes for the Marines.