It's clearly shown DS has a travel time as seen in
this panel (I didn't notice it until he pointed that out) and the panel later where FF warns Sonic before it hits him. Unless Murata's portrayal of how the DS works was flawed, and the slash was actually meant to have infinite speed. The current evidence suggests the slash must travel to reach its targets, meaning it might not "ignore distance" in a way that makes its speed infinite. It could just mean Void can attack you regardless of your whereabouts in the multiverse thanks to his Multiverse Perception.
As for Blast referring to DS as a mere greeting from Void, it can just mean Void has more powerful hax in his arsenal of abilities.