Curiously, here in Zummerset, the postal service mostly improved during the worst of the Covid crisis in that deliveries arrived at 10am rather than lunchtime, and sometimes on Sunday too! Pure luck. My sister in nearby South Gloucester suffers frequent delays. She knows a postman well: he says the difference is mostly caused by the sorting office: my postal area has a spacious modern facility, hers is old-fashioned, due to be modernised, and the cramped together staff have all been ill.
I don't know how the Post Office are dealing with staff shortages due to Covid, which is still causing considerable trouble. I imagine they recruited lots of temporaries who will be laid off as the situation improves. This laying off may be the source of Social Media comment, though I'm sure the PO (as a private company) is always actively seeking to improve efficiency, and not necessarily in customer friendly ways.
Also in the news is the shortage of HGV Drivers caused by leaving the EU. There are about 600,000 jobs and 100,000 vacancies are causing noticeable delivery delays in some areas. Round here, Builders Merchants seem to be worst effected, plus a few gaps on supermarket shelves, but otherwise no obvious effect on my post, Amazon or other deliveries.
I suspect the impact on deliveries due to COVID and staff shortages depends on where you live.
Dave