The "Speed Box" (I call 'em Quick Box cause that's the Brand in my RV but Speed works too) are punch down connectors.. That is the still insulated wire is "punched down" into a metal slit that slices through the insulation and makes a Knife Edge contact with the wire.... Good for Phone (Few thousands of an ampere) up to around single digits.. But once you get into double digit amprage they scare me.. I've melted one, and seen others melt as well.
I really do not like 'em
My Stranded to romex are wire nuts.. and even those have failed on me and needed soldering (As noted above) Soldered does not fail.
For Space heaters and even my "Kitchen 2" circuit.. I bent 12ga wire around screws and TIGHTENED them down.. No problems to date.