Наши три предложения из комментариев, сравнительные:
>>> from timeit import repeat
>>> setup = 'a = [[1,2], [3,4], [5,6]]; b = [[2,1], [4,3], [6,5]]'
>>> min(repeat('[i[::-1] for i in a]', setup))
0.7484351999999959
>>> min(repeat('[[y,x] for x,y in a]', setup))
0.553510899999992
>>> min(repeat('for s in a: s.reverse()', setup))
0.30734420000000284