You have several problems regards speed.?

24V isn't enough voltage you really need around 40-45Vdc to get any decent speed from those motors. I'll Also take a guess that they are high inductance motors which makes things worse with low voltage.

Then you have 5mm pitch screws which will limit your rapid speed.

But to get a better idea of your setup we need more info.
Are you using parallel port.?
Are you using direct drive to screws or do have belts with reduction.?
What micro stepping on the drives.?
What values do you have in Motor tuning. :ie Steps Per, Velocity, Acceleration for each axis.?

When you say slow how slow exactly what's the Max rapid you can achieve.?