Unsure about monzo delays but there definitely is delays with the post, but I would send high value cheques with a tracked service which are running more on-time.
From 19 March, Royal Mail standard post, including Freepost, will take longer to arrive. Royal Mail are prioritising items posted by special delivery, but they’re not guaranteed to arrive by 1pm the next working day.
If it’s urgent, we recommend sending your cheque by Royal Mail’s ‘Special Delivery Guaranteed by 9am’ service, which is still guaranteed. Once we’ve got your cheque, it takes 4 working days to put the money in your account.
The postage times I understand, it was more is there a delay at Monzos end as staff could be self isolating, just trying to prepare myself for how long it will take to be paid in.