One of the great things that Monzo promised back when the original Alphas was announced was a full-stack banking experienced that avoided all of the things that slowed down old banks and made use of the full range of data available from POS and payment systems.
I’d assume that what we’re seeing here (and people have reported the same thing with payments - see Receiving Salary in CA) is that companies build their pay dates around the fact that the legacy banks process slow! Monzo just happens to process the data much quicker 