As illustrated HERE on Astronomy SE, since the velocity of the meteor or comet nucleus is going to be about 40 km/s (essentially the escape velocity from the sun at at 1 AU) and the velocity of Earth is about 30 km/s, the relative velocity of the two can be anywhere from 10 km/s to 70 km/s (varying by a factor of 7) depending on the direction the object is coming from.
The kinetic energy of the object is proportional to the square of the velocity, so it varies by a factor of 50! All of the kinetic energy is converted, in milliseconds, to heat in the air (which quickly become visible and UV light) - or to forming a crater if it reaches the ground. Examining the damage, something of an estimate of the energy can be made, but this could be a smaller object coming in relatively fast, or a larger object coming in relatively slower. This means the mass of the object is very poorly constrained.
The Tunguska aerial explosion is thought to be from an object coming in at a shallow angle. This matters for two reasons: objects coming in at such an angle is more likely to explode before impact, and when they do, they leave a butterfly shaped pattern of damage under them owing to the explosion happening along a path segment rather than at a point.
Light from the explosion, not the fireball itself, reaches the ground essentially instantaneously. The explosion itself takes milliseconds. The light from the plasma created by the explosion fades quickly. Most of the light happens within a small fraction of a second. This light pyrolyzes, rather than burns, the vegetation, carbonizing some of it. Only in the case of a fireball barely above the surface would thr fireball itself interact with the surface.
The shock wave, which knocks over the trees and cools the trees to the point they will not burn, reaches the ground directly under the blast travelling at the speed of sound, in 1 to 3 seconds, depending on height. Ground some distance away from the center is hit a little later, also depending on distance. Trees directly below lose branches but stay standing, similar to Genbaku Dome under the atomic bomb at Hiroshima.