a belt/pulley setup may not be the answer to your problem...
For a more accurate answer before you start throwing money at your machine, listing your current setup would help greatly.
PSU voltage & amperage?
Motor config (Uni-polar, Bi-polar, series/parallel) ?
Motor voltage ?
Motor torque (Oz/In or Nm) ?
Driver config (Amperage settings, Full/Half/Micro-stepping...) ?
Leadscrew type & pitch (ACME, allthread US/Metric etc) ?
Software setup (Mach, TurboCNC etc. motor tuning settings) ?
All of these factors need to be taken into consideration to get the most out of your machine. Maybe your current settings are not optimized at the moment and this is what is producing low speeds.
I am by no means the right person to offer you advice, as I'm still wearing my L-Plates, but the above info is necessary for those experienced members on the forum, in order for you to tune your machine to it's fullest capacity.