> Not any more quickly than the Earth would fall into the existing Mount Everest.
Of course it would happen more quickly. Everest is kept separate from the rest of the Earth by atomic bonds. But an Everest-mass black hole, created at rest on the Earth's surface, would fall through the ground to the center of the Earth very quickly (a few minutes of travel time), gobbling up all the matter within at least a few atoms distance of it's path. Then it would shoot past the center, sailing up toward the other side of the Earth. It would oscillate back and forth, effectively in a damped orbit about the center of the Earth, accreting more and more matter.
Since the accretion rate is probably proportional to the cross-sectional area of the black hole, and the area is proportional to the mass, the black hole size would grow roughly exponentially, at least until it came to rest at the center of the Earth. At this point, it would start accreting as fast as the liquid iron or whatever can get pushed into, which I imagine becomes very fast very quickly.
> an Everest-mass black hole, created at rest on the Earth's surface, would fall through the ground to the center of the Earth very quickly (a few minutes of travel time)
Actually, no, it would take about 45 minutes--half the time it would take to make one orbit of the Earth at approximately the radius of the Earth. That's assuming that the hole's motion is approximately equivalent to free-fall motion, which is what you're basically assuming--no inter-atomic forces exerted on the hole.
> gobbling up all the matter within at least a few atoms distance of it's path.
Not necessarily. Even though there are no inter-atomic forces preventing atoms from falling into the hole, the hole's gravity is still very small, so the force drawing atoms into the hole is very small. The hole's passage will disrupt the forces between atoms in the horizontal direction, but those forces are negligible anyway; the main force exerted on an atom that's part of the Earth is in the vertical direction. So the atoms just to the side of the hole's passage will start out at rest, and they will only fall into the hole if the hole's gravity is enough to pull them sideways into the hole in the time the hole passes.
Just to run some numbers, suppose the hole's mass is 10 trillion kg (my rough back of the envelope estimate of the mass of Mount Everest). Its horizon radius will then be about 1.5 times 10^-14 meters, or about the size of a large atomic nucleus. A typical atom is about 10,000 times larger, or about 10^-10 meters in radius. Using standard formulas for black holes, the time it takes for an object to be pulled into the hole from rest at 10^-10 meters away is sqrt(r^3 c^2/2GM), which works out to about 10^-8 seconds for r = 10^-10 meters and M = 10 trillion kg. But the time it will take the hole to cover 10^-10 meters, and thereby move out of the way of an atom being pulled in from the side, is only 10^-10 seconds at a slow speed of 1 meter/second (which the hole will achieve a tenth of a second after it starts to fall), and about 4 x 10^-14 seconds at the speed it will have at the center of the Earth (about 8000 meters/second, the same as low Earth orbital velocity). So pretty much the only atoms the hole will eat are the atoms directly in its path; its gravity will be too small to pull in atoms from the side quickly enough.
Of course it would happen more quickly. Everest is kept separate from the rest of the Earth by atomic bonds. But an Everest-mass black hole, created at rest on the Earth's surface, would fall through the ground to the center of the Earth very quickly (a few minutes of travel time), gobbling up all the matter within at least a few atoms distance of it's path. Then it would shoot past the center, sailing up toward the other side of the Earth. It would oscillate back and forth, effectively in a damped orbit about the center of the Earth, accreting more and more matter.
Since the accretion rate is probably proportional to the cross-sectional area of the black hole, and the area is proportional to the mass, the black hole size would grow roughly exponentially, at least until it came to rest at the center of the Earth. At this point, it would start accreting as fast as the liquid iron or whatever can get pushed into, which I imagine becomes very fast very quickly.