I have a similar set of experiences (fatigue hiking up hill) and I blame it on metoprolol. 2 years ago I had 3 stents placed in arteries that were 100% and 95% blocked. I have recovered from my heart attack and have been taking a low dose (12.5mg once a day) metoprolol since. My heart rate is slow and steady and seems to be limited to about 90bpm. I am told that the low heart rate is good, however my heart seems to not be able to raise above 90 when I exert myself like walking up hill or need to climb stairs. I regularly walk my dog on 5 mile or longer walks and whenever we go up hill my legs get heavy and I feel fatigued and the feeling subsides when we return to level or down hill walking. I set up my bicycle for aerobic training (as suggested by a cardiologist) and observed that I could not work my heart rate over 100bpm without significant effort and noted that when I had stepped up cycling effort my heart rate would drop before increasing with a corresponding period of being short of breath and feeling exhausted. In short, I experience temporary fatigue when I need to exert my body and I don't like it and I attribute this to metoprolol.

I have discussed this with my primary cardiologist and others I had asked for second opinions and thier directions were similar. A low heart rate is preferred relative to the chance of an Afib related stroke. I did experience Afib 2 years ago and have not experienced it since.