I have recently changed several coil sleeves and waxed the plunger and it makes a Big difference. Especially on older machines.
What may look like a good working mechanism, can be improved by a new sleeve and waxing.
Less friction anywhere on a mechanism will improve it's movement.
Of course you start with the flippers, but also changing the sleeve and wax the plunger on the outhole, kickers, and score reels can make a big difference.