Do you get rust stained water out of this tub/shower valve? If so, they may have used iron nipples or fittings somewhere in the installation, and those are now filling up with rust. If the flow is poor through the valve body with the cartridge out, you've got a supply problem, not the valve. You may have to make a hole from the back to see what's going on. If that isn't a possibility, you may have to tear up the finshed wall in the shower. It's possible that crud dislodged when the water was off could have clogged the cartridge, but it should not be a problem with the new one. Remove the cartridge, then turn the water back on. Flush it out. It will spray everywhere, but should go into the tub instead of in the wall. If you can't get good flow there, you'll have to tear more apart to find the restriction.
Some valves have filter screens burried in them. Those could be clogged. Your tub spout won't , but your showerhead might as well. If those are clogged, it can decrease the flow considerably.