I think your requirements are quite contradictory; my old Commodore 64 had a lean operating system or basically no operating system at all. Yet it took me nearly half an hour to load a single 40 kilobyte game from tape (only to see it didn't work so I had to try loading it again).

What I'm saying is you're not after a lean operating system - you're after fast I/O and that you can get with money. Also the lag you mention is not about operating systems, it's about apps. My "ls" startup is pretty fast:

  $ time ls
  Desktop  Documents  Downloads  Music  Pictures  Videos  bin  code

  real    0m0.006s
  user    0m0.000s
  sys     0m0.000s