Currently there's one underspeed setting for solid infill.
If you make it slow so that you can have a super-nice top layer well all those internal layers of solid infill at the bottom and top of the part are going to bE VERY SLOW TO PRINT.
Ideally there would be an underspeed setting for "solid infill "and an underspeed setting for "external solid infill" i.e. top layers of any plateaux in the part.
So underspeed for solid infill might be 90% and underspeed for external solid infill 40% (just the outermost layer).