I've always been vaguely aware that z moves seemed to happen rather slowly on my Ultimaker, and this was especially true when I enabled Cura's 'Hop on moves' feature - I would often get blobbing, and the moves seemed to introduce noticeable pauses into the printing process. I made some enquiries in the forums and among other users about what z speed they use, and more generally what a good maximum z-speed is. Consensus was that probably 10, 20, 30 or more mm/s was quite sustainable as a z-speed - and yet the defaults in Marlin and Cura seemed very conservative - around 3 to 5 mm/s. My own testing showed that for large moves of the z-axis in non-printing scenarios, 60 mm/s was quite doable, and perfectly repeatable. And yet... Z-moves during printing still seemed rather slow. For a 0.1 mm layer height change, at 30mm/s, simplistic math says that the layer change should take 1/300 of a second. But just looking at the printer, that wasn't the case. A move that fast should look pretty much instantaneous to the naked eye. But while my z-moves were happening pretty quickly, I could just about track the movement with my eye. In the course of all this, I started becoming familiar with the Marlin firmware, and indeed, found and fixed a bug that affected retraction speeds. In the course of all of this, I came to understand much better how Marlin manages the moves that it is asked to do, and particularly the role of acceleration in the motion planning module.