Reason unknown. The rise was present before
    commit d5b5d488
    Author: Ian Lynagh <ian@well-typed.com>
    Date:   Wed Dec 12 17:12:13 2012 +0000

    Use ByteString rather than FastBytes inside FastZString

    Slow nofib Compile Times difference looks like just noise:
    -1 s.d.        -2.9%
    +1 s.d.        +2.9%
    Average        -0.1%
however.