From Dave G3YMC

2.  It has for a long time been the custom to use relatively slow speeds for
conventional CW on 136, in the belief that this is better when signals are
weak.  I would question the wisdom in that.

I have lost a number of QSO:s because the operator at the other end
was sending much too fast with respect to the circumstances.
I have no problem with QRQ if the conds allow it.

It also is confirmed in the SM7CMY report that lowering the speed (in general) 
improves the readability.
If I remember correctly, 5 wpm is given as optimum speed when signals are weak.
However, I don't think that static chashes were included in the test routines...

/sm6pxj