Children in Kent showing signs of ADHD, autism, dyslexia or learning and communication disorders are spending two years without a formal diagnosis.
Parents or carers could be missing out on disability living allowance or educational support for their child because of the wait.
But head of mental health commissioning in Kent and Medway, Dave Holman told councillors the time it takes to get a diagnosis for a neurodevelopmental disorder has halved since 2016.
At Kent County Council's health overview and scrutiny committee, he said: "It was four years but now the average wait is two years, which is clearly far too long.
"This isn't just about diagnostics, this is also how you support children once they have had their diagnosis as well."
However, 69% of children wait more than 18 weeks to get their first full assessment with the neurodevelopmental and learning disability service.
Mr Holman, who works for west Kent clinical commissioning group, explained demand for these services is high but he is confident about the progress of the provider, North East London NHS Foundation Trust (NELFT).
"I'm really confident this organisation...Is doing its best to reduce the waits and meet the demand which is huge compared to what it was a couple of years ago..." Dave Holman
He said: "We are seeing people more quickly now but clearly there is still more work to do.
"From a commissioning point of view, I'm really confident this organisation, in very difficult circumstances, is doing its best to reduce the waits and meet the demand which is huge compared to what it was a couple of years ago."
However not all of the councillors on the committee are convinced this is good enough.
At the meeting on Friday, Cllr Nick Chard (Con) said: "NELFT says it is trying to achieve that the patient should not wait longer than 18 weeks. They should try a lot harder.
"What concerns me most is NELFT is not a group of pleasant, well-meaning volunteer people, they are paid to do it.
"They are paid well to do their job and they are clearly not delivering."
Cllr Karen Constantine (Lab) shared concerns children on the waiting list are missing out on their education, social development and it is damaging their relationships with their parents or carers.
She said: "The report was quite shocking because clearly it does indicate there is a lot of gaps and young people that fall through them, that's obviously very worrying.
"When you are waiting for access to a service, one of the things that suffers is school and education.
"What worries me is the opportunity for that young person to catch up.
"Also, the interface between the parent and the carer tends to decline and often can come a really critical situation.
"People have reported to me their children have become violent towards them.
"The other thing that alarms me is the ability of young people to make and form relationships with people, which sets them back."
"They are paid well to do their job and they are clearly not delivering..." Cllr Nick Chard
West Kent CCG's clinical lead for mental health, Dr David Chesover, explained while "insufficient" money is a factor to the team not meeting this pressure, more children are being seen within the required time.
He added: "The frustrations felt by councillors are felt everyday by GPs and clinicians in the system and the management trying to support them.
"People are working extraordinarily hard the whole time to revise the system on limited resources."
Dr Chesover also shared his concern things are a lot harder for children now than in his 40-year career practicing medicine.
He said: "Children are now absorbing more of society's stress.
"There is a much more complex, fundamental insecurity and less relience in children than there has been in the 40 years I have practiced medicine.
"This is a really complex issue."