Suggestions for controlling steppers in multiple axes (with A4988)?

Hello! We are trying to control steppers in 6 axes using multiple A4988 stepper drivers. We’re particularly interested in using Arduino to control the drivers.

We’ve already unsuccessfully tried using the Adafruit Motor/Stepper/Servo Shield for Arduino (it didn’t move quickly enough for our purposes). There are several CNC shields which use multiple axes and interface with an Arduino for this purpose, which we have been considering.

Does anyone have recommendations for multi-axis control? For example, have you had success with Raspberry Pi versus Arduino, or do you recommend any shields in particular?