Question of understanding: If what you say is true, and it actually does take up to three days to transfer the securities, how can you immediately sell an asset that you just bought a few minutes ago? My understanding of HFT (including from your description) was that this type of thing, i.e. holding an asset for only a few minutes, happens all the time. How can that be possible if the transfer takes as long as you claim it does?
But then yummyfajitas' comparison doesn't apply after all. When transferring money, the recipient has to wait a day until he can order the next transfer. This is not the case with trading, so this whole "three days" claim seems spurious to me.
I think his point was that the time it takes to be agreed to and the time it takes for the trade to clear are two different things, and that it's OK for those to happen on different timescales.
Having the trade agreed to quickly is a big deal even if it could take days to have any tangible assets to show for it.
