If the wax ended up squeezing out and partially blocking the drain, that could be the problem. I much prefer to fix the flange so it is the proper height. And, I'm not a fan of the funnels in the wax ring. If that was off-center a little when you set the toilet, it can partially close up and block the output. It can also be problematic on some flange designs. An alternative to fixing the flange is to use a waxless seal - I've used the one from Fluidmaster. That will accommodate the recessed flange. So, it might not be the toilet at all.
If you take the weight off the flapper, it will release more water. Or, you can hold the handle down, which could dump the entire tank (a lot more than 1.28 or 1.6g), you'd get the max. I'd pull the toilet up and verify things are clear, and maybe consider the Fluidmaster seal; then see what happens.