http://elitistjerks.com/f31/t15257-m...ighlight=parry
The average reduction in weapon speed is 25% on a parry. This isn't the whole story since a parry immediately following a melee swing is a much bigger reduction in weapon speed (up to 40%) and is the usual source of parygibbing, so your reduction of .25% per point is mostly valuable in preventing that from happening moreso than any real tangible amount of mitigation.