This was documented on our docs as "Transaction Delay - Delay in waiting for unterminated commit acknowledgement, in milliseconds.". The main problem was not the counter but is the understand of what is exactly is measured by this counter.
The solution to this problem is that you need to divide (Transaction Delay) / (Mirrored Write Transactions/sec) to get AVG DELAY. This means customer had 15 seg of wait for ALL transactions on that measured second
The first value is an accumulation of the delay of all the current transaction delay in millisecond
The second counter is the value of transactions which indeed caused data transfer with AlwaysOn (in opposite to SQL Server:Database – Transactions/sec which does include pure read transactions as well).
In order to get to the average transaction (in principle commit delay) one only has to divide both values. In the screen shot below we look at an average Transaction Delay of 392 milliseconds (green line in 0.1 scale) and around 60 Write Transactions/sec (red line in scale 1.0). Calculating the numbers, we look at a transaction delay of around 6.5ms.
After found the information below I requested an update to official doc to avoid future confusion