I'm pretty new to this, could you give me some general figures about achievable speeds?
No it depends on the motor, the driver and the power supply.
but every now and then has a hiccup where it appears to stall for fraction of a second
Sounds more like the software than the power supply.
Note you also loose a bit of voltage with that chip so your 9V is probbly closer to 6V. Upping the supply to 14V would give you more.