Three elements determine the quality and “performance” of a bicycle: 

1. Light weight: It’s easy to understand what a determining factor the weight of your bike can be when you ride uphill: the force of gravity becomes the cyclist’s principal obstacle. 
In truth, weight is also important during accelerations, not only for the obvious ones when you bolt from a group in an attack or to go into a sprint, but also for the ‘micro-accelerations’ that occur with every uphill pedal stroke, especially at reduced speed and pedal cadence. 

Five hundred grams saved on the weight of the chassis and non-moving parts translate into an advantage of about 30” for every hour of climb; an even more determinant factor is the weight of wheels, crankset and pedals: every 100 grams saved correspond to an average gain of 20” per hour of climb. 
The rotational inertia of the crankset, wheels, and pedals resists every angular acceleration of these same moving parts, with an added energy cost depending on their mass. 

2. Rigidity: try to imagine pedaling on a bike made of rubber: your efforts would only bend and distort the pedal arms, chassis and wheels, with precious little of your power being transformed into forward motion. 

Therefore, chassis, crankset and wheels need to be rigid in order to deliver the highest possible percentage of power from the pedals to the ground. 

Construction materials and chassis geometry will be critical, along with the material used for welding: for the most part, rigidity depends on the material used, but also on the thickness and diameter of the tubing.

 

Also, using materials with different torsion strength can permit construction of an equally rigid chassis by increasing the tubing section (as is done in aluminum chassis with oversized tubing). 

In general, carbon fiber chassis are particularly rigid, aluminum chassis are very light, titanium chassis are the most comfortable, steel the most stable. 

3. Comfort: If you have to stay in the saddle for a long period, comfort can be the most important consideration: it all depends on chassis geometry and the manufacturer's experience. 

Even wheels can add or detract from the bicycle’s comfort factor, for example, in the choice of tires with regard to road surface.