Watford to Luton is via non-motorway roads, but, it has taken me over half an hour to drive through the centre of St Albans on a Sunday, and this was a Friday afternoon. The driver was pleasant and apologised but blamed the 'usual Friday traffic'.
Another passenger waiting at Luton said that 'it was a regular occurrence for buses to 'not appear' then turn up together'.

The service is run from the Watford depot, and one of the Luton depot drivers said that they have no control (a bus has to arrive in Luton before it can depart - there are no spares available at Luton)..

Luton is in the process of moving all bus arrivals/departures to the Railway Station Interchange from the streets in the centre of town (Silver Street), but, no doubt, some services will still run through the centre (the 321 serves Church Street).

In future (for my once a year visit) I will stay on the 321 to the Railway Station and catch my connecting 24 or 25 from there (I believe) instead of walking through the Arnedale Centre to Silver Street. It might save time - providing the 321 turns up for the return trip.

The 321 was half-hourly until last year (but is now three-times per hour - introduction of Sapphire?).
Still frustrating when they are delayed.

The 321 also used to run through to Luton Airport, but Arriva decided not to pay the charges demanded by the Airport to terminate there (as an example passengers are charged £2.50 for dropping off by car and up to £20 when collecting if the flight is delayed) and airline passengers now have to change buses at the Railway Station - with extra charges for those who pay of course.