Replacing the chain I found that the plastic wheels of the rear derailleur are rotating with high resistance.
So I disassembled the wheel, cleaned it from dirt and sprayed with WD-40. Here is how it looks now:
I have Motul Tech grease 300, that I used to lubricate the ball bearings in the hub. So, should I put the grease between the steel cylinder (how is it called?) and the inner round hole of the wheel?
Or it will just attract dirt and make things worse?