You tried squirting some lube on the pistons? Might be a bit sticky?
Eh? Sticky pistons wouldn't cause poor braking performance, they would result in rubbing brakes, or the pistons wouldn't pump out properly, meaning the levers would hit the bars before the brakes are fully applied.
If you think your pistons are sticky, you should remove the pads, pump the pistons out as much as you dare (about 6mm?) and apply a drop or two of brake fluid to the piston sides, then push the pistons back in, and repeat until smooth. If the pistons are so sticky they won't pump out and just retract when you release the lever, hold one piston in whilst pumping the lever to extend the other.
Don't use normal lube, and definitely don't squirt it 🙂
Isopropanol can be got from Maplins or chemists, though you may have to convince the chemist that you have a legit use for it. I have also used automotive clutch/brake cleaner from halfords.