If the prospective circuit then meets the timing objective, you can make further investment to legalize the placement.
When experimenting with circuit timing, there is no permanent risk from experimentation that temporarily breaks the circuit to collect a data point.
Add registers in illegal locations to determine the effect on overall timing.
When there are many nodes on a path, the Fitter must place nodes away from each other, resulting in significant routing delay.
In contrast, a circuit that you heavily pipeline is much less dependent on placement, which simplifies timing closure.
More complex circuits, like barrel shifters and crossbars, grow super-linearly as bus width increases.
If you cut the bus width of a barrel shifter, crossbar, or other complex circuit in half, the area benefit can be significantly better than half, approaching quadratic rates.After you embed components at a higher level, the block speed may be the same.However, the speed may never be any faster with other components than alone.Addressing the timing closure initially with margin is much easier.Timing closure of a slow circuit is not inherently easier than timing closure of a faster circuit, because slow circuits typically include more combinational logic between registers.Use realistic timing margins when creating your design.