I'm a Klipper user and tend to print at highish speeds (100-150mm/s).
In my experience, have no problem printing Solid Infill at 100% speed.
Typically have 5 layers of solid infill going down on top of the infill (typically fast honeycomb). It is ONLY the first of those 5 layers that I'd like to apply underspeed to and only when on top of some regular infill: because the layer is effectively bridging the gaps of the honeycomb structure and this needs to be done more slowly for good results. (If I forget to slow it down, there will be lots of bumpy filament fragments on that first layer.)