The comment I replied to suggested that a speed up was to use an SPV wallet, and I replied with information regarding the risk if someone was to get such an 'implementation', not a discussion of the 'protocol'.
So not all that hard to get ...
A > B > C > D > E > F
> D'
where D' is the fake block. The SPV client is capable of noticing this and following the longest chain. If the attacker continues building on D' it still is going to be a lot shorter than the real chain and won't be followed by the SPV client.
It's the 6.25BTC number that matters, and if some scoundrel company with more than 6.25 BTC (which there are plenty of companies with a lot more than that) wants to take advantage of an implemented SPV wallet with a lot more than 6.25BTC in it, where the SPV wallet only talks to them ...
I wonder why a mod deleted one of my posts and who asked for that ...