Some measurements importing a gzip-compressed file (zcat .. | ncdu -f -)
containing a bit under 6 million items and a few choices of how often to
call input_handle():

Called on every item:

  real    0m13.745s
  user    0m12.576s
  sys     0m4.566s

Called on every 8 items:

  real    0m7.932s
  user    0m9.636s
  sys     0m1.623s

Called on every 16 items:

  real    0m7.559s
  user    0m9.553s
  sys     0m1.323s

Called on every 32 items:

  real    0m7.279s
  user    0m9.353s
  sys     0m1.277s

Called on every 64 items:

  real    0m7.166s
  user    0m9.389s
  sys     0m1.117s

Called on every 256 items:

  real    0m7.073s
  user    0m9.439s
  sys     0m1.027s

32 seemed like a good compromise.