Limits of Round the World Travel
I ran across an interesting website, Wingnet.org, with the history of around the world travel.
Just for fun, I plotted the times of various milestone flights over the years.
My conclusion: We really have not made any significant advance in flight speed technology since the 1930's. The world speed records have really not changed significantly since Howard Hughes 1938 speed record of 91 hours. There are two graphs shown here: one is linear scale, the other is log scale so you can see the more recent values.
I have left out the speed of orbital flights. John Glenn broke the RTW speed record in 1962 of 4 Hrs and 55 mins. It seems that orbital travel is the only way to get another major improvement in speeds.
Have we reached the 'Human' limit of earth atmospheric travel?