I'd be surprised if this was causing the bad shifting, especially on a newish bike. I've messed with old Shimano derailleurs that have given years of service in the wet and mud of Lancashire with constant soakings in mucky water and washings and I've always been amazed at the smooth, play-free action of the springs and paralellogram pivots. They seem to be plain pins pressed into the alloy derailleur body and since the movement isn't much they seem to last forever. The only bits of Shimano derailleurs I've ever needed to refurbish, apart from the pulley wheels, is the main frame pivots and once, the cage pivot.
As I've written above, my money is on a worn or dirty or rusted cable outer inhibiting the shifting or a bent hanger, both problems I'd expect to inherit with a second-hand bike.
