currently multisigs will delay time because the front end currently take a long time. due to having to manually build a transaction.
bt if the devs (im not relying on core dvs because they dont care about such things) othre devs will make userfriendly front ends that make it simplistic and easy as typing in an amount and pressing send.
thus a milisecond thing. not a minute thing.
imagine it.
you have a multisig with an exchange. the LN node is api calling the exchange as a hub, to send the signed tx's
along with signed TX the LN node can obviously API call other data to the exchange.
so imagine your LN node is the exchange 'bot' thus not needing to log into an exchange webpage separately but all done within an LN node
This means that I need to send data to Bitstamp three times when using LN, verses only sending data to Bitstamp once if the BTC is confirmed into my bitstamp account. This might not take very long, but it would be long enough for someone to see that I am about to trade and act accordingly.
i am still not completely familiar with LN but still this data that you are talking about is a transaction (half signed transaction to be exact) so the size of it should not be anything more than 250-300 bytes (unless it has lots of UTXO) and even sending/receiving it 3 times with slowest internet connection shouldn't be any trouble. and making the transaction is of milliseconds and signing it is also as fast.
to everyone else, please stop repeating things like "exchanges security itself", "Don't keep your coins on an exchange", .... they are off topic here.