Massive slowdown in mwc-random after switching to use of primitive package

[Simon Peyton-Jones]

|  > caused a huge performance regression.
|  
|  You're using GHC 6.12.x presumably? There are known performance problems with
|  using abstract PrimMonads in that version (and, actually, just using IO as
|  well). 
...
|  The good news is that, last I checked, this isn't a problem in HEAD. The
|  optimizer does better there somehow. The bad news is that 6.14 isn't out yet,

This is the first I've heard of this.  Do you have a test case that shows up the problem?  Then we can put it in the regression tests so it won't go wrong again.

Simon