There  are approximately two billion children (persons under 18) in the world.   However,  since  Santa  does  not visit children of Muslim, Hindu, Jewish  or  Buddhist  religions,  this reduces the workload for  Christmas night  to  15%  of  the  total, or 378 million (according to the Population Reference  Bureau).  At  an  average (census)  rate  of  3.5  children per household,  that  comes  to 108 million homes, presuming that there is  at least one good child in each.

Santa  has  about  31  hours  of  Christmas  to work with, thanks to the different  time  zones  and  the rotation of the earth, assuming he travels east  to  west  (which  seems logical). This works out to 967.7 visits per  second.  This  is  to  say  that  for each Christian household with a good child,  Santa  has around 1/1000th of a second to park the sleigh, hop out, jump  down  the  chimney,  fill  the stockings, distribute the remaining presents  under  the  tree, eat whatever snacks have been left for him, get back  up  the  chimney, jump into the sleigh and get on to the next house.  Assuming  that each of these 108 million stops is evenly distributed around the  earth  (which, of course, we know to be false, but will accept for the purposes  of  our calculations), we are  now talking about 0.78 miles per household; a total trip of 75.5  million miles, not counting bathroom stops or  breaks.  This  means  Santa's sleigh is moving at 650 miles per second 3,000  times  the  speed of sound. For purposes of comparison, the fastest man-made   vehicle, the Ulysses space probe, moves at a poky 27.4 miles per second, and a conventional reindeer can run (at best) 15 miles per hour.

The  payload  of the sleigh adds another interesting element.  Assuming that  each  child  gets  nothing  more  than  a  medium sized Lego set (two pounds),  the sleigh is carrying over 500 thousand tons, not counting Santa himself.  On  land,  a  conventional  reindeer  can pull no more than 300 pounds.  Even granting that the "flying" reindeer could pull ten times the normal  amount,  the job can't be done with eight or even nine of them Santa would need 360,000 of them.  This increases the payload, not counting the weight of the sleigh, another  54,000 tons, or roughly seven times  the weight of the Queen Elizabeth (the ship, not the monarch).

600,000  tons  traveling at 650 miles per second crates enormous air resistance.  This  would  heat up the reindeer in the same fashion  as a spacecraft  re-entering  the  earth's atmosphere. The lead pair of reindeer would absorb 14.3 quintillion joules of energy per second each. In short, they would burst into flames almost instantaneously, exposing the reindeer behind  them  and  creating deafening sonicbooms in their wake. The entire reindeer  team  would  be vaporized within 4.26 thousandths of a second, or right about the time Santa reached the fifth house on his trip. Not that it matters, however, since Santa, as a  result of accelerating from a dead stop to  650  m.p.s.  in  .001 seconds, would be subjected to centrifugal forces of 17,500  g's.  A  250  pound Santa (which seems ludicrously slim) would be pinned to the back of the sleigh by 4,315,015 pounds of force, instantly crushing his bones and organs and reducing him to a quivering blob of  goo.

Therefore, if Santa did exist, he's dead now.