Servicing/tensioning a rear mech I have a Tiagra 4600 10 speed rear mech (mid length) which is sluggish to shift into higher gears. 

Shifting was all over the place, I’ve replaced the jockey wheels and checked the hanger alignment. Shifting is now indexed fine up the cassette but lags behind every click when shifting into harder gears, when the return spring is doing the work. It’s indexed ok this way too, but lags a full cog every click. 

Is there any way to service a rear mech, to increase the spring tension? I could source a replacement spring or other parts off junked derailleurs at LBS if there’s a solution to be had there.
For a full diagnosis, remove the chain, and unbolt any cabling. With just a hand, push the RD body in and out. As long as the RD is free to move and returns back in line with the smallest cog and retains some mild resistance to inward pressure when lined up with the smallest cog, then it's less likely an RD body spring. 
Try shifting the derailleur by pushing in on it while you pedal and then letting go. If it shifts well then the spring is not terribly weak. Now try wrapping a rag around your fingers and pulling up on the shift cable at the chain stay and then at the down tube and note any change in shifting to the small cog. If it slows down a lot then you've identified where the friction is, and it is friction that is most common cause of shifting problems down the cassette.  Given the new and lubed cables and housing you're dealing with, how about a closer look at the shifter? Perhaps cleaning and lubing that will fix the issue.