That is a shallow minimum; both λ = 120 and λ = 160 gave vector lengths of 986, and the length slowly increased with greater λ.
the fastest bit-count on this architec- ture, using a technique first published by Wegner (1960), requires time increasing with the number of nonzero bits it counts; and a similar effect would appear on a word-by-word basis even if we used a constant-time per-word count.