Calipers will always have a little drag. Even my 4 pots drag a little.
I would recommend demounting it and cleaning the sliders and relubing and assessing from there. If the seals aren't leaking, they don't need to be rebuilt. It's a sliding 2 piston caliper. If the slide isn't smooth, it'll bind.
Pending that, Bike Bandit will have a rebuild kit or at least buy the o-rings in the fiche.