I think you're missing a basic point Nick.
Landing clearance can
not be given if an aircraft (or veichle) is occupying the runway. That means that the preceeding aircraft must completely have left the runway area before the next is cleared to land.
In a late landing clearance situation (like in the video) the preceeding is late off the runway, this means the controller has to hold the clearance until the runway is free.
Since only one person can transmit at once (we'll leave technical details out for now I think) the controller needs to make sure nobody else is transmitting when he needs to give landing clearance.
This means that when the preceeding is close to vacating the controller will start his transmission and then drag it out, so he can give the clearance the second the runway is free. That is the reason you may hear things like
" [speaking slowly] callsign, surface wind is calm, runway 23 Right, you are [normal speed] cleared to land"
This means that the aircraft gets it's clearance as soon as possible while making sure nobody else transmits at the critical moment thereby forcing the go around (causing delay, unnessecary noise and also burning fule).
So the reason for the slow transmission is the fact that they are waiting until they may acctually give the clearance and to make sure that nobody prevents them from giving clearance to land.
Did that wall of text do anything?
