The Welsh Liberal Democrat leader, Kirsty Williams, has called for a review of the salary paid to the chairman of the Welsh Ambulance Trust.
She says Stuart Fletcher is among the highest paid ambulance service chairs in the UK, in a £40-45,000 pay band.
That was despite the Welsh trust missing a target of getting to 65% of serious 999 calls within eight minutes.
A Welsh government source said ambulance staff should be praised for their hard work, not attacked.
"These figures paint a very telling picture of how the Welsh Labour government runs our NHS and, indeed, how it spends Welsh taxpayers' money," said the Brecon and Radnorshire assembly member.
"We are not getting the best value for money. The chair of the Welsh Ambulance Trust is paid the highest salary despite the trust having the lowest response rates, by a significant margin, when compared with the other 12 ambulance trusts in England and Scotland."
According to the Welsh trusts' own figures in its annual report for 2011-12, its ambulances made it to 68.4% of the most serious emergencies within the eight minute time, describing it as "the best performance by the Trust since its formation in 1998".
But the latest figures released at the end of the year, show that in November, the category A call times had dropped by a full 10 percentage points.
"I find it outrageous that we are financially rewarding failure and poor performance," added Ms Williams.
"Paramedics and staff at the Welsh Ambulance Trust provide a very important service under difficult circumstances.
"They must find it hard to stomach the amount paid to their chair when they see that his counterparts across Britain are performing better and paid less."
According to the Lib-Dem figures, only one other ambulance service chair is in a £40-45,000 pay band, at the South Western Ambulance Service. It made it to 75.9% of category A 999 calls within eight minutes.
The mid-point for salaries paid to ambulance service chairs is the £20-25,000 band.
Outside of Wales, the lowest percentage for responding to the 999 calls was for the London Ambulance Service, at 73.2% arriving at serious cases within eight minutes.
But a source in the Welsh government dismissed the attacks from Liberal Democrats.
"The Welsh Ambulance Service is working extremely hard in difficult weather conditions, and we should be thanking ambulance staff, not attacking their performance," said the source.
"It's testament to the hard work and dedication of staff that emergency response services have been delivered throughout the bad weather, despite dangerous driving conditions."
A review of the Welsh Ambulance Service is currently underway, and is due to end in March.
The government said it would deliver recommendations to "enable high quality, sustainable ambulance services for the future".
The government source added: "In making this attack, the Welsh Liberal Democrats are nothing but hypocrites - pure and simple. If they want to make comparisons between the Welsh and English NHS, then they're skating on very thin ice indeed.
"They cannot on the one hand prop-up a Tory Government that is causing utter mayhem to the NHS in England and then attack the health service in Wales. It's two-faced to say the least."