OP, I share your history of myocardial infarction. Before my heart attack more than 15 years ago I routinely averaged 20 miles in one hour and logged on a few occasions 50 miles in less than 2.5 hours. Since then I have never bested 16.5 mph over 20 miles and normally average about 15.5 mph for that distance.

Certainly there is unresolved heart damage and scar tissue. My ejection fraction is not awful, but at 45% is below normal for a fit athlete. Not surprisingly I take a beta blocker daily. It is difficult to get my heart rate above 115 bpm. Normally I top out around 120 bpm. At 65 years old I should be riding comfortably at 85% of (220-65) or 132, but that is really difficult for me, most likely due to the beta blocker I think.

These two factors, the heart damage and the beta blocker are surely responsible for the decline in my cycling performance. My race ready road bikes at 13.5 and 14.2 lb weight don't provide any help to me to regain my prior form. I am simply stuck in this reduced performance rut.

So if you ask me, it is the engine, not the bike that is causing your problem. In case you are wondering, I was cycling quite strongly when the MI struck. In fact I was drafting a gorgeous young woman at the time. Trust me, it is hard to keep grinning with an elephant sitting on your chest!

Keep trying. You may see some improvement. Just don't think you can turn back the clock on a heart attack.