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Wales News Service
Only just over a quarter of coronavirus tests at drive-through centres were processed within 24 hours last week.
It is the worst performance for samples taken at the 11 regional centres since the pandemic began.
The latest statistics show that 26.8% of the 5,252 tests at the centres came back within a day, while 86.3% were back within two days.
The Welsh Government said it was putting measures in place to improve turnaround times.
The centres are mainly used by the general public and are based in Cardiff, Newport, Abercynon, Cwm, Llandudno, Deeside, Carmarthen, Swansea, Margam, Builth Wells and Newtown.
The performance of smaller community testing units (CTUs), where healthcare workers are given priority, improved slightly, with 36.6% of around 8,000 tests being processed within 24 hours.
Overall processing is still not being done as quickly as the Welsh Government wants it to be.
First Minister Mark Drakeford has said he wants to accelerate and simplify the system.
Experts advising the government say the best contact tracing systems rely on results coming back within 24 hours.
Of the 4,500 tests done in hospitals, 87.9% did meet that target.
A Welsh Government spokeswoman said: "As set out in our recently published strategy, testing turnaround times is a key focus and we have been putting in place measures to improve our turnaround times in the labs and throughout the end-to-end process."
The Welsh Conservatives said the statistics showed "just how inadequate" the Welsh Government's testing response has been.