Bosses at East Midlands Ambulance Service Trust said the organisation is “not where they want it to be”, despite an improvement in performance.
The trust improved its response times to life threatening calls since March 2018, despite missing its targets.
Sue Cousland, general manager for EMAS in Lincolnshire, said there was a long way to go for the organisation.
“We’re not anywhere near where we would want to be,” she said.
“We’re not compliant with our performance trajectory and there is a lot more work to do.”
The ambulance trust missed its targets for both category one, made for life threatening situations, and category two calls which are made for patients with emergency conditions, such as chest pains or burns.
EMAS took on average eight minutes and 45 seconds to respond to a life threatening call, which is an improvement on ten minutes and 39 seconds last year.
But, the response time still missed the target of seven minutes for a category one call.
Meanwhile, the trust responded on average 33 minutes 25 seconds to a category two call against a target of 25 minutes.
The time is an improvement on 49 minutes 44 seconds in March 2018.
Councillors on the health scrutiny panel for Lincolnshire described the performance as “refreshing”.
They added the targets set for ambulances do not reflect the challenges they would face in Lincolnshire, such as demographic and rurality.
EMAS was given an extra £9 million in funding in March 2018 in an effort to help improve its performance and help it to respond more quickly and consistently to 999 calls.
National response targets are set to change from April 2019.
Ambulances will be expected to reach category one calls on average within seven minutes and category two calls within 18 minutes.