If we compare properly designed IWB holsters against properly designed concealment belt rigs, there is little speed difference. If both are designed to hold the butt tightly against the body and allow a full firing grip upon initial hand contact, their speed will be equivalent. With most IWB's, the shirt-tail frequently starts to interfere with reholstering after several presentations have been made. While this may be a nuisance during a shooting class, it is of no consequence on the street...where, if needed at all, you'll make ONE draw.
Some makers try to minimize the "shirt-tail interference" problem of IWB's by putting a larger "shirt guard" behind the upper portion
of the pistol. These generally interfere with getting a good firing grip upon initial hand contact and I do not like them.