As another user mentioned, many git actions scale linearly in the number of changes, not in the size of the repository. Try recreating the scaled repo, but say, in commits of 1000 lines each (ie. 200K commits), and see how long things take.
Did your experiment also do 40,000 changes per day (35 million commits, of varying sizes throughout the repo), and then see how that affects git performance? My (admittedly crappy) understanding of git is that it also scales on the commits, not just the raw file number/size count.