Mansfield actually builds a pretty decent toilet. I'm doubtful it's
the toilet. But here are some things to check.
Use a closet auger on that toilet. If the end of the auger comes back
with any wax stuck to it, it means whoever installed the toilet got
that wax seal a little off-center and it's restricting your flow. The
auger will also remove any other types of blockage.
Also check along the lip in the toilet. There are a bunch of little
holes all around the underside of that lip. Use a small nail and make
sure all of those holes are clear. Also make sure the jet in the
bottom of the toilet is clear (the small hole...not the waste hole)