What is the speed with which electrons revolve around the nucleus in its orbits?
That presupposes that electrons are little hard particles for which you could measure their speed. Sorry to burst your bubble, but that isn’t the case. At the atomic scale, energy and matter blend together so that in one experiment an electron may look like a particle (photoelectric effect) and in another it may appear to be a wave of electromagnetic energy as described by De Broglie and Schrodinger. That is the nature of the quantum theory. Quantum theory is best understood from the math, not by trying to visualize an object moving under the rules imposed by Newtonian physics. Therefore, at best you could say that electrons can approach the speed of light.
