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Why is a road bike faster than a city bike with the same effort? and how much faster it is? (if someone have measured it)

Motive of the question:

My current bike is a city bike with front fork shock absorber & back shock absorber under the saddle, it is very heavy & I feel maintaining 20 km/hr for 30 minutes is very hard on flat city roads, I am thinking of switching to a road bike but I want to understand if it is really worth it as my bike is very sturdy, with hub dynamo, fenders & rear rack (all of these are not on the entry level road bikes)

I know this sound a bit controversial but I hope someone will help me from our community

Thanks

Let's assume you are talking about speed on a non-inclined surface for a given level of effort. (When we talk about how 'fast' a bicycle is we can also mean how well it accelerates from a standstill or low speed, or how well it handles around turns or on a bumpy surface.)

Constant speed is achieved when power applied to the pedals is equal to the power lost to overcoming aerodynamic drag, rolling resistance of the wheels and all other losses in the bearings, drivetrain, frame and wheels flexing etc.

Aerodynamic drag increases with the third power of velocity so at higher speeds it dominates. One of the biggest difference between a 'city' or 'hybrid' bike and a drop bar 'road' bike is a more upright riding position which results in a larger frontal area and a much higher drag.

City or hybrid bikes also typically have wider tires run a lower pressures which have higher rolling resistance than narrower, higher pressure tires typically seen in road bikes.

On bikes with suspension cyclic compression and uncompression of the springs and dampers due to unevenness of pedalling force and movement of the rider also soaks up energy.

Your hub dynamo will also be adding some resistance.

Having made the change myself, I can confirm that shock absorbers are actually detrimental to city riding. You lose a lot of power, especially when trying to stand on the peddles for acceleration.

Road bikes are also typically much lighter, which in my experience not only helps you go uphill faster, but also makes it a lot easier to carry the bike when necessary. For instance when I am leaving my place, with my old bike I would've had to clumsily roll it out, but with the road bike I can just carry it out to the street. It's not something you typically think about, but it saves time.

As a final note, while you can't turn a mountain bike into a road bike, you can make some changes that would improve it's road performance a bit. One thing you can do is swap out your normal nubby tires for slicks. Slick tires, pumped to the appropriate pressure, will reduce your rolling resistance. Also if your shocks are the adjustable kind, then increase the firmness, or even lock them entirely, so that they won't waste as much of your energy.