the entire length of inner cable should have been greased before putting it together, it may not be possible now if cable is too bent/frayed to come out & go back in. Hold the cable up and drip light machine oil down inner till it runs out other end.
This may not be your problem tho as it is usually caused by folks putting too much bend into the cable - let the whole thing hang freely and see if it is better.
This has been covered in other threads, but here it is again. You probably need to take the lever off the shaft and move it over a couple of splines. Lots of these kits come with the lever way out of line with the cable, which increases pulling force considerably. Just by getting the lever in line with the cable, will make a huge difference, plus it helps keep the lever tucked in and not hitting your leg wwhen you pedal.