And no, the slower miners don't see any negative impact, because we accept shares, and pay them for shares for (CurrentBlock) and (CurrentBlock - 1). If a block is solved every 600 seconds, it would take them 1200 seconds to have their new shares for a getwork become stale.
That is not what is stated in the OP.
- Anti-fraud protection. We only accept shares from the current block, in the current round.