With a .9 degree step angle I think your torque vs speed curve is even worse. Our top speed is probably double yours. You need to find the torque curve specs for those steppers. we drop off at 60rpms, you probably drop off at less. those are high accuracy steppers not high speed. You might just be fighting a power/rpm issue.
This should give you an idea of what I am talking about. https://www.vicious1.com/software-updates/