I have replaced the cable and housing on a road bike with ST-5700 and RD-5700. The cable is slick stainless. If I activate the shift lever when holding the cable where it exits the first housing portion, it is extremely smooth. Same where it exists the frame, same again where is exists the housing that ends in the rear derailleur. Cable movement is perfect, up or down.
As soon as I attach it to the RD-5700, cable pull is messy and hard, indexing is impossible. Chain is new, tune up was done, hanger was aligned and so on. I took special care to not give the cable a curve or a bend. The last 3-4 clicks to pull the cable are VERY hard.
I've encountered this problem with this derailleur many times this season on many of my customers's bikes. The issue is always the same. I've triple check the cable attachement spot is good.
My only option left is to test it with a complete full length housing and give up the internal routing, I'm not sure the points where the end caps touch the entrance and exit of the frame are very flat, but that wouldn't explain the difficulty to pull the cable when operating the lever.
The derailleur itself has no play in the pivots and movement is no hindered by static friction.
Another idea: are we supposed to use plastic housing where the cable exits the rear housing before it gets attached? I reused the hold one in this case, didn't change anything.
Also, I wanted to check what shimano says by the books, but there are no dealer's manual for this model.