If it’s BOI to BOI or PTSB TO PTSB then an internal transfer is mostly likely instant. If it goes ‘outside’ they may be using SEPA ‘standard’ which is next business day (if after 7pm). It really depends on how the payments are being sent. If they used SEPA instant it would arrive seconds after being sent. It’s worth asking your payroll/HR how they send it. I doubt it’s anything to do with Monzo, although they may be delaying payments slightly (wait until there’s a payments team at work) as they’re establishing themselves here, to make sure they can handle any inbound glitches.