The problem at Cardiff is how many generic messages they have over their PA. There is one about smoking, unattended luggage and ‘see it say it sorted’ that are probably needed, then you have ‘there are ticket buying facilities at this station...’, ‘this platform slopes towards the track’, ‘due to essential engineering works...’, ‘Penalty fares now apply across the Great Western Railway Network...’ an occasionally very long specific engineering works ones too, which take up quite a lot of time in two languages. Some of these could probably be discarded.
I think announcements queue for 30 seconds at Cardiff and then are wiped off. I’ve also never heard any announcements for any trains ever on the concourse except for platform alterations, pretty sure it’s just generic safety messages only?.
The generic announcements should only play when there's nothing else, so - in theory - they shouldn't delay anything. But I guess that once they start, they must finish and this can therefore cause a delay if there's an unexpected announcement (like a delay or platform alteration).
From what I've seen, manual announcements will also cause platform announcements to be delayed, unless it had already started in which case a manual announcement interrupting will cancel it altogether. This means staff doing announcements may need to time things carefully.