I have integrated Bullet physics to my engine and I am having trouble with the timestep. I have read the "Fix your timestep" and other articles regarding this issue, but none of them help me with my design. This is my update function:

int SFMLWindow::run() {
        bool bRunning = true;
        sf::Clock clock;
        sf::Time oldTime = clock.getElapsedTime();
        sf::Time deltaTime;
        while(bRunning) {
            
            sf::Time currentTime = clock.getElapsedTime();
            deltaTime = currentTime - oldTime;
            currentTime = oldTime;
            
            mRenderWindow.clear();
            mRenderWindow.pushGLStates();
            mCurrentState->update(deltaTime.asSeconds());
            mRenderWindow.popGLStates();
            
            mRenderWindow.display();
        }
        return 0;
    }

The state holds all the systems and passed the "deltaTime.asSeconds()" parameter to the update function of these systems. And here is the bullet physics update function:

   void BulletPhysicsSystem::update(double dt) {
        mWorld->stepSimulation(dt,10.0);
    }

The problem is, the sphere I have in my scene goes with a speed of light to the ground. What should I add or do?

I cannot believe I missed that! Thank you!

Yes thats what it does. Here is the wiki from Bullet Physics:

It's important that timeStep is always less than maxSubSteps*fixedTimeStep, otherwise you are losing time. Mathematically,

timeStep < maxSubSteps * fixedTimeStep

When you are calculating what figures you need for your game, start by picking the maximum and minimum framerates you expect to see. For example, I cap my game framerate at 120fps, I guess it might conceivably go as low as 12fps.

After you asked the question, I couldn't stop myself from digging into the source code of Bullet:

if (maxSubSteps)
	{
		//fixed timestep with interpolation
		m_fixedTimeStep = fixedTimeStep;
		m_localTime += timeStep;
		if (m_localTime >= fixedTimeStep)
		{
			numSimulationSubSteps = int( m_localTime / fixedTimeStep);
			m_localTime -= numSimulationSubSteps * fixedTimeStep;
		}
	} 

So it looks like the "semi-fixed timestep" from the article I was referring to.