I think the question should be changed to "I believe the biggest obstacle to global economic growth OVER THE SHORT RUN is . . . "

Populism, geopolitics, trade wars, rising interest rates, etc. are all recent factors to the slowing economies.

I believe the most fundamental reason of slowdown in economic growth OVER THE LONG RUN is due to low birth rate and aging population. Japan has already shown the world. In the not-so-far future the world will be like Japan, we will have a very slow growing world economy on an absolute basis (though GDP per capita, i.e. the living quality and productivity of each person, will continue to improve).