Are bicycles with wide tires bad for road racing? I have Schnell Damping 26, which is a mountain bike with front and rear suspension. Some people suggest me that thick tire bikes are not good for road racing. What is the reason? Also, some suggest that back rear suspension is not good and causes a lot of sweat.
Wide tyres are designed to be pumped to a relatively low pressure so that they bend around objects they roll over, to give grip and a smoother ride. They also often have lots of bumps and lumps on them: again, these dig into the ground and bend around things for more grip. However, bending and deforming all this rubber hundreds of times a minute takes a lot of energy. The only source of energy on a bike is you, so this means that a lot of your energy is going into the tyres, rather than driving the bike forwards. Sometimes, you hear a mountain bike with really knobbly tyres being ridden on the road and it sounds like a swarm of bees: that's a lot of wasted energy.

Suspension (front and rear) is, by design, flexible. This means that, when you press on the pedals, some of your energy goes into driving the bike forwards, but some of your energy goes into flexing the suspension.

Roads are relatively smooth, which means that fat tyres and suspension aren't giving you any advantages. But they're still there, using your muscle power for useless things. They're also heavy, which makes it harder to accelerate your bike and harder to go up hills. These are significant disadvantages to using this kind of bike on the road. Another disadvantage of mountain bikes is that they often have very low gearing, which means you can't go very fast, even spinning your pedals as fast as you can.

Some bikes with suspension have a control that lets you stop the suspension moving. That would be a good thing to use when riding on the road, but you'll still be carrying around all that excess weight and your squishy, fat tyres.

And, in fact, you might not be allowed to ride a road race on your particular mountain bike. The international regulations of the UCI require something fairly close to a traditional diamond frame. The intent was to exclude some of the extreme aerodynamic designs that people were experimenting with, but a mountain bike with rear suspension would also probably be forbidden under these rules. Although any race you'd be likely to enter wouldn't be run by the UCI, local cycling federations usually use rules very similar to the international ones, so that the rules don't suddenly change when people turn pro.