> "Trades are agreed to in milliseconds"

One might ask "why is this important if the transfer itself takes days?"

The main reason is that it gives you certainty and closure. You try to make a trade for + or - X shares of Y at $Z and you get an essentially instant response. You don't have to wait around all day just to know if you've got a deal; you don't have to poke around looking for a slightly better price; you just make the offer and see it accepted immediately, and can move on to other things. (This is the sense in which I meant the word "clear" a few posts up: the trade is cleared from your to-do list, even if the securities take some time to actually move.)

There is a cost for the service of being able to move on: the HFT will likely capture a tiny bit of profit which could have gone to you with more effort. You've traded money for time and effort.

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?

You don't actually have to have the securities physically in hand in order to enter into a contract to transfer them.

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.