I've always been a proponent of the theory that, "It's the archer, not the bow".
Taking this to riding and here's my influence list. High influence to low influence:
- human motor
- wind
- riding surface
- bike weight
- ambient temperature
- bike type
- tires
- quality of components such as wheels & bearings
- bike age
- other
As far as personal performance on rides, that's a fickled thing. One day a ride can feel effortless and another day it's a real drama. Diet may have a big influence on performance too.