It is not speculation - it was and is obviously true if one reads history without an agenda or an ax to grind.
I will leave it to others to explain how a serious illness affects judgment although one would think it is self-explanatory.
I've read several books on Gettysburg and several biographies of Lee and while there was some speculation on his health during the summer of 1863 not blamed his decision on the third day on a heart attack.
Primary sources from those who were with Lee and knew of such an infirmity flaring at Gettysburg? Citations from reputable historians?
If so i'd be real interested in seeing it. Please link them here if you have them handy. --Thanks.
What is known is that Lee did have bouts of what we would today call Angina Pectoris, but no contemporary evidence that he suffered any such attacks during the Gettysburg Campaign exists. If you have such evidence, please point to it. Again, thanks ahead of time.
He did suffer from what appears to be dysentery during the Gettysburg Campaign, but appears to have recovered by the third day.
Other men, even commanders have served remarkably well under similar circumstances. The first name that would come to mind is Major General Frank Merrill of the famous Merrill's Marauders in the China-Burma theatre of WWII. A close second, maybe even first, would be Brigadier General Theodore Roosevelt Jr.