In-house legal aid lawyers who are paid an annual salary are more efficient than private lawyers funded by Legal Aid NSW and paid according to an hourly rate, according to new figures from the Bureau of Crime Statistics and Research.

The research covered 34,218 criminal law grants of legal aid from 2012 to the end of 2016, revealing that the type of representation provided to a defendant "substantially impacts" the way their matter proceeds through the criminal courts.

Legal Aid NSW requested the research over concerns that a "billable hours" system and low fees paid to private lawyers handling legal aid work creates an incentive to maximise the number of hours spent on a case, meaning a resolution is delayed.

"We found people are less likely to plead guilty [with a private lawyer] because the case is going to be finalised faster than if they plead not guilty," BOCSAR executive director Don Weatherburn said.

"[A private lawyer] will go to a higher court because there is more money than if you finalise in a local court. The fee structure works as an incentive to delay a case."