Print speed is the setting everyone reaches for, and it is not the thing that limits a printer. A hotend can melt a certain volume of plastic per second, and that limit is measured in cubic millimetres a second. Print speed is only one of the three numbers that decide how much the slicer is asking for.
flow (mm³/s) = line width × layer height × print speed
A 0.42mm line at 0.2mm layers running 120mm/s asks for 10.08 mm³/s. Against a typical 12 mm³/s hotend that is 84 per cent, so there is room but not a great deal. The maximum volumetric speed calculator also reports the other direction: at that width and layer height, the fastest the hotend can feed is 143 mm/s.
Three dials, one limit
Because the three multiply, a change to any of them is a change to all of them. Raise the layer height from 0.2mm to 0.28mm and leave the speed alone, and the demand goes from 10.08 to 14.11 mm³/s, which is 117.6 per cent of the limit. The fastest allowed speed drops from 143 mm/s to 102.
Fit a 0.6mm nozzle, run 0.3mm layers and keep 120mm/s, and the figure is 21.60 mm³/s, 180 per cent of the limit, with a ceiling of 67 mm/s. That is the usual disappointment with a larger nozzle: it does lay down more per pass, and almost none of the expected time saving survives, because the hotend was the constraint all along.
What it looks like when you exceed it
The extruder keeps commanding filament and the melt zone cannot supply it, so you get under-extrusion: thin walls, visible gaps in top surfaces, weak layer bonding, and in bad cases the extruder grinding or skipping. It looks exactly like a partial clog, which is why people clean a nozzle that was never blocked.
The test that separates them is to print the same part slower. If it comes out clean, the nozzle was fine and the flow was not. It is also worth seeing the figure from the filament side: at 10.08 mm³/s the filament itself is feeding in at 4.19 mm/s, and extruders have their own mechanical limit independent of the melt rate.
The other cause of under-extrusion
Before blaming flow, check the calibration. If the extruder is told to push 100mm and only 96.5mm goes in, every print is 3.5 per cent short, and the steps per mm calculator turns that measurement into the corrected firmware value, 96.373 from a starting 93. On a 50 gram print that error is 1.8 grams of missing plastic.
Fix it in firmware rather than by raising the flow multiplier in the slicer. The multiplier exists to account for a particular filament, and using it to hide a machine error means the error comes back the moment you slice in a different profile.
Once both are right, the remaining questions are time and money: print time estimate for the first, and filament weight and cost for the second. For resin, where the constraint is layer exposure rather than flow, resin print cost covers what a part actually consumes once supports and failures are counted.