Hertfordshire has lost more than a fifth of its firefighters since 2010, while crews are taking longer to respond to emergencies.
The Fire Brigades Union has lambasted the "appalling cuts" to fire services across the country, which it says are putting public safety at risk.
In the Hertfordshire Fire and Rescue Service, the number of full-time equivalent firefighters fell from 802 in 2010 to 629 in 2018 - a drop of 22%.
At the same time, crews took 37 seconds longer to respond to callouts in the 12 months to March 2018 than in the same period in 2010 - a 7% increase, bringing the total response time to 8 minutes 50 seconds.
The figures refer to primary incidents, which are the most serious fires with potential to harm people or cause damage to property.
Across England, the number of firefighters has fallen by more than 22% since 2010, from 41,632 to 32,340, and there are now 45 fewer fire stations.
The average time taken to respond to serious incidents increased by more than 30 seconds over the same period.
The Home Office said local factors could affect response times, and that there is not a straightforward link between response times and the outcomes of a fire.
However, Matt Wrack, FBU general secretary, said there was a clear link between slower response times and government cuts.
"Year on year we are seeing appalling cuts to the service and these figures are clear evidence that the cuts have gone too far," he said.
"Understaffed fire stations across the country struggle to provide a 24-hour service to their community, with the starkest effect outside of cities.
"Fire and rescue services are expected to do more with less and it is only down to the dedication of fire and rescue staff that the service is performing at all.
"The service is at breaking point. Public safety is being put at risk.”
Response times in Hertfordshire are slower than the England average, which last year stood at 8 mins 45 secs.
The slowing responses in England have come despite the fact that firefighters are attending fewer primary incidents.
In 2009-2010, there were 1,549 in the Hertfordshire area, compared to 1,048 by 2017-18.
There has also been a fall in the number of fire stations operated by the service, from 30 in 2010 to 29 in 2018.
A Home Office spokesperson said: “The Government is thankful for the continued tireless efforts of firefighters across the country.
“There has also been a long term downward trend in both fires and fire deaths for many years, recently reaching historically low levels, and we are confident that fire and rescue services have the resources they need to do their important work.”