For what it's worth I'm taking a 20mg simvastatin. I can't say I've noticed any effects on performance.
Although I recently as of Feb really started working out again. I started with push up and crunches working my way up to 50+ consecutive push ups and 100+crunches. Started running on the treadmill at work, and have been cycling. I've only experienced a decrease in my resting heart rate. Increased performance and last night I ran my best mile time of under 12mins, getting close to my goal of 8mins.

If you have just started taking it maybe you need to give yourself an adjustment period.