15.7.6.2 For Pipelined Logic, Place Registers on Region Boundaries

If you assigned logic to a region so its inputs and outputs are bounded by a register array (pipeline registers), it is a good idea to place these pipeline registers close to the boundary of the region. If you plan to manually fix the placement of your pipeline registers, make sure you orient them in the correct direction to assure a smooth data flow between them and their interfacing logic.