Then this paper should be wrong:
http://people.scs.carleton.ca/~clark/biblio/bitcoin/Karame%202012.pdf

"In this paper, we analyze the security of using
Bitcoin for fast payments, where the time between
the exchange of currency and goods is ... seconds).

We focus on doublespending attacks on fast payments and demonstrate
that these attacks can be mounted at low cost on
currently deployed versions of Bitcoin.

We further show that the measures recommended by Bitcoin developers for the use of Bitcoin in fast transactions are
not always effective in resisting double-spending; we
show that if those recommendations are integrated
in future Bitcoin implementations, double-spending
attacks on Bitcoin will still be possible.

Finally, we leverage on our findings and propose a lightweight
countermeasure that enables the detection of doublespending attacks in fast transactions."

I haven't read it all yet, I am curious about the "lightweight countermeasure" they purpose...

But this proves that the one saying "this is slow" had a point!

So what do you reply now?