Block generation is determined by previous blocks.  If you are doomed to generate next block in 3 hours you can prepare a block instantly, but you have to wait for 3 hours before you can reveal it (otherwise it will be ignored by the network).  You can build millions blockchains in parallel to find the best one for an attack, but at some point you will need someone else to generate a block (or wait again).  Nxt uses a novel PoS algorithm, not the one PPC, NVC and other altcoins do.  Nxt doesn't have their flaws.