<p>If a customer sends me a non-commercial 3D model to print, am I allowed to charge money for the 3D printing process? I understand I cannot charge anything for the model nor offer it as a part of my business.</p>

<p>I cannot download the model myself, print and sell it, but if the customer downloads it and sends it to me for me to print it, is it a violation of the license or not?</p>

In more practical terms, you could design the part so that the corners are rounded (also known as fillets). This will help keep the print head moving and would prevent the sudden stop and start effect that causes "jerking". Further 8 bit controllers tend to get saturated when reading large amounts of g-code from the sd card or the serial port. Upgrading to a 32 bit controller will prevent that kind of jerking.
Both of these methods pale in comparison to just speeding up the print. Upgrading the hardware to be faster (various methods exist) would yield more of a reduced time than trying to optimize the g-code (in my humble opinion). Delta printers have the potential to be the fastest type of FDM printer, assuming that you could get the filament to melt fast enough.