The most important with the first layer is that it sticks to the bed. Therefore it is also important that it´s easy to control the print speed. As I see it, the speed should always be default printing speed whether it is Raft, Infill, Outline, Skirt, Prime pillar or something else. The speed control for this layer is set under first layer settings. To complicate things a bit and increase adhesion even more without dropping the speed too much, an option for lowering the speed even more for the first 10mm or so after rapid moves.