I'm afraid, guys, that I'm probably not going to be experimenting with VFD settings, at least not for my current project.

I wasn't sure that there was a lack of torque - pushing a 2.5mm cutter, even through steel, isn't going to need very much - but rather a problem with speed control at low speeds. Watching the cut in progress, I suddenly remembered what the effect reminded me of. I was seeing what was almost a tiny pause, a slight release of the cutter, and associated squealing and poor finish. Just like turning when you take a shallow cut on a metal prone to work-hardening. So I wound up the feed speed a little which helped significantly. I then ran out of the cold-rolled bright mild steel I had been using and swapped to hot-rolled black steel. Which cut much better and without the vibration/"pausing" effect - so I think I probably found the answer. I suspect that the bright mild knocked the edge of my cutter, though, as it didn't last much longer even in the softer metal. I'll ramp up DOC next time and see how it goes.