East Midlands Ambulance Service (EMAS) failed to reach thousands of patients in rural parts of Derbyshire in 2013 within the target time, figures show.
EMAS is supposed to reach 75% of serious cases within eight minutes.
But in areas around Matlock, Ashbourne, Swadlincote and Bakewell, figures suggest the service failed to reach even 50% of people within that time.
The service said sparsely-populated areas presented it with "many operational challenges".
BBC Radio Derby asked EMAS, in a Freedom of Information request, for its average response time to Red 1 or Red 2 emergencies in each of the postcode areas that it covers.
Across Derbyshire and parts of East Staffordshire in 2013, the average response time was seven minutes and 14 seconds for Red 1 calls.
For Red 2 calls it was six minutes and 46 seconds.
But the average response time in many rural postcode areas was much longer.
Calls for improvement
In the Ashbourne, Matlock and Bakewell areas, the most serious emergencies were responded to in an average of about 11 minutes.
In the DE12 postcode area, near Swadlincote, the average Red 1 response time was more than 10 minutes.
The service's overall average response time - on which it is judged by the Department of Health - was improved to less than six minutes by fast average response times in Derby.
Heather Wheeler, Conservative MP for South Derbyshire which includes Swadlincote, said: "The service has not been good enough.
"I have had the opportunity to meet with the new interim chief executive and interim chairman and they assure me their number one aim is to give a decent timely service to the people of South Derbyshire."
EMAS said: "When compared to urban areas, the number of calls we receive is very low.
"This means there is a reduced likelihood of there being a vehicle in close proximity to where help is needed.
"In response to these challenges, we provide cover by placing staff at strategic standby points rather than them waiting to respond to a call from a station which may itself be a long way from the location of the next call."
In May 2013 EMAS was fined £3.5m after it had missed response time targets for a third successive year.