Yes, they are slower drying, but if you paint a thick layer of OPEN acrylics for the underpainting (we want this to be opaque) you might actually have to wait longer than if working with the Alkyd quick drying oils before applying the next layers. And if you painted the underpainting with OPEN acrylics and then applied standard acrylics on top, you run the risk of the upper paint layers drying before the OPEN acrylics underneath.