Hi, glad to hear your dh is recovering and yes, I'm afraid you're being impatient 
My step-mother, and a friend's dh, had heart attacks a couple of years ago and both were shocked by the tiredness afterwards - it took weeks, and rushing is really not a good idea. I think my friend's dh had at least a month off work, if not more, and my step-mother (a true workaholic) took a slower pace for at least 6 months.
The medics treating your dh should be able to advise on heart damage, and he should get follow up care and heaps of advice - if you're not getting enough advice, do go to GP or back to cardiac department and demand some. But tiredness is NORMAL after a heart attack, and not necessarily anything to do with damage - my step mother had virtually none, and my friend's dh very little, but they were exhausted.
Plus, if he's on beta blocokers (which he probably is at the moment), those can make you feel tired and slower - if he's ok after some time, they may lower the dose and/or stop them, so long term good news but, I'm afraid, more patience required.
HTH a bit, and that all goes well with ongoing recovery!