To my experience (and if only the filename is needed) the "-1" option helps saving some time too. Be aware that output from ls without options differ, if redirected.

time (real user sys) in seconds with 1e6 files in a xfs filesystem. Directory used 27 MB space.

  ls -l > /dev/null     : 49.12 23.90 13.47
  ls -lU > /dev/null    : 46.62 33.11  8.01
  ls > /dev/null        :  6.36  1.97  3.63
  ls -1 > /dev/null     :  2.49  1.83  0.36
  ls -1U > /dev/null    :  0.31  0.18  0.08
  (find)                :  0.77  0.49  0.19

and for 1e6 files in an ext4 filesystem. Directory used 23 MB space. (but as i had no ext4 fs with one million free inodes on a loop device)

  ls -l > /dev/null     : 51.39 34.02 11.39
  ls -lU > /dev/null    : 47.39 30.72 11.09
  ls > /dev/null        :  4.83  3.56  0.65
  ls -1 > /dev/null     :  4.46  3.45  0.50
  ls -1U > /dev/null    :  0.51  0.18  0.26
  (find)                :  1.29  0.76  0.36