I've been surprised that my similar frames can feel so different, but the question was about components, so getting the saddle (and its location, which adds seatposts into the discussion) right makes the biggest difference in comfort and hence, my "performance" - more than handlebars or their reach, height, width, etc. (although I haven't used anything but drop bars yet).

Lately, I've been trying different but similar tires (Pasela and Grand Bois of the same size) and can definitely feel differences.

And I have to agree with legs, lungs, heart - components of the engine - and body weight.