An elementary but not obvious fact of milling: a machine that's weaker cannot just simply go slower. Every time a cutting edge of a spinning mill bit comes around, it has to be far enough advanced into new material to actually cut and carve off a chunk of material. Otherwise it's just rubbing, and maybe ablating some material, but mostly making heat rather than progress. Trying to remove material that way quickly turns into too much hotness for either the cutter or the material. Like, stirring a little puddle of melted acrylic. Or if the spindle motor can't even make enough power to heat stuff, then nothing happens.

So there is no "slow" in milling. The machine either can push the cutter through the material fast enough to push each edge into new material, or it can't.

Cognoscenti call this "making chips".

My ignorance of this was a critical enabling capability for starting this project. Because clearly there's no way a CDROM sled can shove a mill bit through anything harder than floral foam.