Why is ++posting_list_iterator slow?

[elshize]

Documents only: 12.3762 ns/p; 80.8002 mln p/s
Documents only nextgeq: 13.0795 ns/p; 76.4558 mln p/s
Documents and frequencies independently: 15.1303 ns/p; 66.0927 mln p/s
As posting_list_view: 15.7871 ns/p; 63.3429 mln p/s