The speedup comes to the fact that the previous revision uses 5 splits as a standard. The splitnumber means how many depthmaps are taken to build the shadow (1 split would be the same as standard shadowmapping).
the values can lie everywhere between the camera nearplane and the farplane. Shadowmapping is a imagebased shadowing approach so you have aliasing effects when you get the depth of a region bigger than the depthtexture. The splitting lowers this aliasing because each split gets its indivudal depthtexture. eg with your values the rendering is split into 4 passes: the firstone renders the depthtexture for the range 1.0 to 4.0, the 2nd will capture 4.0 to 28.0 and so on. you see, the further away the fragments, the lower the amount of space is given per pixel. so accuracy is lower for those further away but higher for the nearer ones.
the values can lie everywhere between the camera nearplane and the farplane. Shadowmapping is a imagebased shadowing approach so you have aliasing effects when you get the depth of a region bigger than the depthtexture. The splitting lowers this aliasing because each split gets its indivudal depthtexture. eg with your values the rendering is split into 4 passes: the firstone renders the depthtexture for the range 1.0 to 4.0, the 2nd will capture 4.0 to 28.0 and so on. you see, the further away the fragments, the lower the amount of space is given per pixel. so accuracy is lower for those further away but higher for the nearer ones.