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Already discussed this with madmike8. You will find it on the bottom of the comments.

These motors can do only 15RPM reliably. With 50mm circumference pulleys that are installed in ToyREP it is 12.5mm/s. Almost all of my 28BYJ-48 steppers can do 30RPM and some hand picked even up to 60RPM when overloaded, but without good cooling they were prone to die very soon. I was running the machine like this at the very beginning, on the first video you can see the machine doing 30mm/s in X and Y axis. The major problem is how to cool down the coils that are hidden inside the motor and barely touching the metal housing ... So trying to make them spin quicker is probably not good way to go.

I was also experimenting with bigger pulley circumference (on different machine as ToyREP is not designed for such pulleys), but 75mm is already dangerously close to the strength limit. With such pulley you can do 18.75mm/s. Two motors with 150mm circumference pulleys can possibly do 37,5mm/s but you have twice as big deviation caused by the internal gearbox and you have to imagine almost 50mm dia pulleys installed in the printer.

Edit: Maybe 35BYJ-46 is better way to go instead of two 28BYJ-48. They can be found fairly cheap too.