Well, exactly what trajectory would you predict for the result of inbreeding between Homo Sapiens and Neanderthals?
For starters, I would predict that Celtic peoples would be among the first to advance, rather than among the last,* since Celtic peoples would be expected to retain the most Neanderthal genes.
* And that almost entirely derivative of non-Celts.
PS. To be more precise, that's the trajectory I would predict in the event that 'accelerated Northern Hemisphere advancement' was attributable to Neanderthal/Cro-Magnon interbreeding. Something else I would predict in that case is for civilization to arise and flourish in those regions first, rather than in the Nile, Euphrates, Indus, and Yellow river valleys. Within Europe itself, I would predict the northwest to be more advanced than the southeast, which was not the case until very modern times.
The only advance, albeit a huge one, that was first achieved in northwest Europe was the Industrial Revolution.