FWIW, a properly build shower pan should not leak even if it was filled to near the top of the curb (the liner is below the tile and some substrate compatible with the tile, so leave maybe 1" or so to the top of the finished curb). Code says no holes in the liner within 2" above the top of the curb, so if that is done right, assuming you plug the drain well, it should fill up and not leak. A slow drain just implies there may be a partial clog, or the slope of the drain or the venting is not done right. Actually, without a vent, it could flow way too well, and not break the suction, and siphon itself.