The Mini 2 has a Service Ceiling of >13000 feet.I fly from 5200 ft MSL, and it seems pretty zippy to me! But at 6200 ft MSL (under 400 ft AGL) the controls do seem to be a bit more sluggish. Not as bad as you describe, but I can easily extrapolate that mine might do the same thing at 7200 ft MSL.
At 6200ft or 7400 ft any altitude effects would have been insignificant and too small to notice.
Altitude wasn't the cause of whatever you noticed.