What size membrane do you have? The units themselves are pretty much all the same, the difference really lies in the cartridges and membrane. If you have decent water pressure (~80psi), just replace your membrane. They're all slow, but anything less than 90 gpd is painfully slow. You'll also need to replace the flow restrictor to match your new membrane.
If your water pressure is is low, then you should definitely consider a booster pump. Also worth noting, RODI's will fluctuate with the water temp, particularly if you're on well water. The colder the water, the slower the unit.