In my experience:
1) when printing a first layer slowly, this is of particular importance for fine, delicate features and the perimeters which go down first, as well as for the ends of lines when there are changes of direction. However it's of lesser importance when laying down large areas of parallel tracks (that offer each other stability);
2) when printing a top layer it can be nice to slow down the solid infill to make it look as beautiful as possible;
If solid infill has been set to 70% and first layer speed to 50% (for the particular benefit of the fine features) it's somewhat annoying that first layer solid infill is then printed at 70% * 50% = 35%. Especially if a part has a large, flat bottom. It slows down the print unnecessarily.
EITHER: make it such that "First Layer Speed" is applied equally (and uniquely) to all first layer features (including solid infill, including support), such that they are all printed at the same speed.
OR: implement something that is even more complex and individually parameterable!!