Dear kind sir, please read the fine manual now in thread number 3. Are you going to keep making new threads along the lines of "how does trivial action regarding timing constraints work?" Read the"Xilinx timing constraints user guide", and all shall be clear. For link, see other thread. Or google title.
But yes, GENERALLY (so not your LEON thingy specifically) if you have a design with some amount of slack for a given clock, and THEN you reduce your clock period then your slack will also reduce and can go negative.