Cardiff, Newport, and Swansea are among the top 20 poorest-performing post code areas for delivery of first class mail.
Royal Mail says the service is improving
Royal Mail has revealed that only two other post code areas in the the UK are worse than Swansea for meeting the target for delivering first class post by noon next day.
The company has failed to meet all 15 performance targets, and is paying £50m in compensation for late deliveries.
But, it insists, the service is improving.
In Swansea, only 81.9% of first class letters arrived on time. Oxford was the worst, with only 68% arriving as expected.
In Cardiff it was 88.6% and in Newport it was 87.6% - both appearing in the top 20 worst performers, although showing improvements since May.
Other parts of Wales made it into the Royal Mail's top 20 best areas - in mid Wales, more than 92% of first class letters are delivered on time.
Neville Sims, vice chairman of Postwatch Wales, said he was "disappointed" with the overall figures and was calling on Royal Mail to improve its performance.
"What we are asking, now that the single daily delivery has come in, is consistency of service," he told BBC Wales.
"People want to know when their letters will arrive."
Gareth Davies, Director of Welsh Affairs for Royal Mail, said poor performance could not be excused, but pointed out that Royal Mail had recently undergone "probably the biggest modernisation" of any major organisation in the UK.
"It is very disappointing from a Welsh perspective," he said. "Swansea is causing us most concern, because it has always been held in high regard in terms of performance, and it is being looked at."
WORST PERFORMING AREAS
Oxford - 68% first class post on time
SE London 85%
N London 85.3%
Royal Mail said the service had seen a major improvement in August, when it rose to 92% from May when just 87.2% of first class post was delivered on time.
Deliveries improved after dipping in the spring, when changes to the service took place.
"There is now real evidence the service is getting better," said Royal Mail chief executive Adam Crozier.
Mr Crozier, who took personal day-to-day control of the letters business in May, added: "Clearly there were some problems in the spring but the service has improved,
month-on-month, since May and we are now approaching our target level for first class mail."
He added that he hoped to hit all targets within the coming months.
Second class post is now hitting its target of 98.5% of items being delivered on time, compared to April, when 98.3% was getting to its destination in time.
In May that fell to 97.9% and 98% in July.
Mr Crozier said the organisation was now making progress, adding that 95% of the operational changes - such as scrapping the second post- had now been made.