I just weened myself off Cymbalta. I was on it for 5 years. During this time I did some bike racing and I discovered that my heart rate Max was abnormally high for a person my age (58). Not only that but if I maintained over 91% of that max for over a few minutes I would feel nausious. Like I'm going to throw up now kind of sick feeling.
Since I've gotten off Cymbalta my heart rate came down to a normal level. And I can go as hard as I want, for as long as I want and I never feel sick. Ever.
So if your taking Cymbalta it does affect your performance.
Bad Bad stuff.