Research by the Labour Party found the force was eight per cent slower at responding to emergency calls between 2012-13.
The study also showed that response times for the West Mercia force increased by 7.1 per cent in the two year period bewteen 2011-13.
The figures were revealed as it was shown the majority of police forces in England and Wales are getting slower.
More than two-thirds of forces to respond to a freedom of information request by the Labour Party revealed an increase in response times to 999 calls.
Among rises over a two-year period, between 2011 and 2013, response times in Devon and Cornwall increased 50 per cent from six to nine minutes, while they were up 31.4 per cent in Kent from 8.4 minutes to 11.04 minutes.
As the Police Federation annual conference entered its second day today, with the impact of austerity measures high on the agenda, Labour blamed the Government budget cuts for the slower police response times.
The number of police response officers have fallen by 8,458 to 37,408 since 2009/10, according to Labour.
Shadow home secretary Yvette Cooper said: "When people call 999 in an emergency, they need to know help will be there fast.
"But instead victims are waiting longer and longer for that help.
"David Cameron and Theresa May said the front line would be protected but the reality is vital response units have been heavily cut back."
Of the 21 forces who replied to the request, 14 showed substantial increases over the last two years, two forces showed a significant increase in the last year, while a further two remained the same.