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I just noticed the other day that my Ender 3 is printing stuff a lot slower than it can, and realized when I was slicing a part that it's because the default "Wall Print Speed" is half the "Print Speed" even though "Infill Speed" is full speed. I can enter higher speeds, of course, but then Cura puts up a little notification that "this value is normally calculated, but it has been entered instead" and offers to put it back to the calculated default -- which is half the "Print Speed."

I presume there are good reasons (print quality, underextrusion, etc.) for this -- what are they?

I guess it's relevant to note that I have a 4.2.2 mainboard and currently run Creality's version of 2.0.4-1.0.1 Marlin, the newest they offer for this board version without BLTouch.

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Cura has a lot of bad defaults, but also a lot of defaults that are chosen to let you get acceptable prints out of a really bad printer, a miscalibrated printer, or a printer with bad firmware.

Printing at anything above very low speeds requires compensation for the differing pressure needed to move material through the nozzle at different flows. The printer firmware feature to do this compensation is known as "pressure advance" or "linear advance", and is absent in stock firmware on most commercially produced printers, especially older ones. Without it, you'll get bulging corners and underextruded middles of your walls. This matters most on the outer wall, which determines the dimensional accuracy and visual quality of your print; errors on the inner walls or infill will mostly be hidden and tend to even out, especially if the inner walls are printed after the outer ones so that the extrusion is constrained against the already-existing outer wall (but here's where Cura's wrong defaults come in: it defaults to printing outer wall last, which replicates the inner wall errors onto the outer wall!).

On a printer without pressure advance compensation, you can use either very low speeds (30 mm/s or preferably even lower) to get a decent outer wall, or you can use high acceleration and cornering velocity (but: most stock firmware doesn't handler cornering velocity correctly either) so that the corners aren't printed significantly slower than the rest of the wall, but at high acceleration/fast cornering you'll get ringing (ghosting) artifacts. These can also be eliminated with better tuning of your printer/firmware, but the point of Cura's defaults is giving decent output on a printer where that work hasn't been done.

If your printer has a version of Marlin (at least 2.0) with linear advance and junction deviation, or if you upgrade to that, or if you switch to Klipper, and then you tune these to match your printer, there's no reason to keep the outer wall speed low. With Klipper you can take the acceleration way up too (using input shaper to eliminate the ringing). This can give you prints many times faster than Cura's defaults. Even if you don't, you can still turn the speed and acceleration way up, but you'll get ugly and possibly structurally weak prints as a result.

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  • $\begingroup$ My printer was new in early May 2021; it has a 4.2.2 mainboard and the most current Creality version of Marlin (2.0.1-1.0.1 is what the download says, though it comes up on screen as 1.0.1). I'm headed toward upgrading to actual Marlin, still working on learning how to get it from downloaded source to in-printer firmware. $\endgroup$
    – Zeiss Ikon
    Jul 9 at 16:35
  • $\begingroup$ I haven't followed what build Creality is shipping these days. If it has linear advance and junction deviation, you can tune it to get good results at fairly high speeds already, without rebuilding. $\endgroup$ Jul 9 at 18:03
  • $\begingroup$ I don't know how to do any of that stuff. Looks like more questions to ask... ;) $\endgroup$
    – Zeiss Ikon
    Jul 9 at 18:04
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Perimeters, especially External Perimeters, are particularly sensitive to small deflections of the printhead, as well as extrusion issues from too fast of a print speed. You may be able to increase this speed for regular perimeters, however, higher speeds may introduce more ghosting issues.

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    $\begingroup$ Perimeters are especially sensitive to sharp turns leaving a wavy artifact ater the turn. $\endgroup$
    – Perry Webb
    Jul 9 at 14:56
  • $\begingroup$ Yes, that's called ghosting. $\endgroup$
    – craftxbox
    Jul 9 at 15:06
  • $\begingroup$ Ghosting is a function of acceleration not speed. $\endgroup$ Jul 9 at 15:52
  • $\begingroup$ Speed is dictated by accelleration. $\endgroup$
    – craftxbox
    Jul 9 at 21:05

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