The Scottish Government has set a target of an eight minute response time for "category A" incidents, which are defined as cases where life is at risk.
However, data obtained by the Scottish Liberal Democrats has shown the response time for the most serious incidents was more than 20 minutes a total of 2,692 times in 2013-14.
The figure is slightly under 1.9 per cent of the 142,340 category A call-outs the Scottish Ambulance Service received over the year.
Liberal Democrat MSP Jim Hume called on the Scottish Government to provide ambulance staff with more support: "Our ambulance service does fantastic, life-saving work in communities around Scotland every day. But I think ambulance service staff would be amongst the first to recognise that these figures are hugely concerning.
"For people suffering from cardiac problems and other serious health conditions, every minute without treatment in an emergency can reduce the likelihood of survival. It is that simple."
He added: "The eight-minute target for responses to serious incidents was introduced for good reason. Ministers now need to look closely at why this it is still being missed, and why so many patients have been forced to wait more than double this time to receive medical assistance."
Glasgow and Peterhead had the highest number of 20 minute-plus response times, followed by Edinburgh, Fraserburgh and Blairgowrie.
The Scottish Ambulance Service responded by saying the target was to respond to three quarters of potentially life threatening calls within eight minutes. The average response time in Scotland was six-and-a-half minutes, despite a 10 per cent surge in demand, a spokesman said.
He added: "Response times can be affected by a number of factors, including demand patterns and weather conditions, but ambulance staff are committed to delivering the highest standards of patient care (and) often do so in extremely challenging situations.
"If for any reason there is a delay paramedic advisors will review the call and provide additional clinical support to the caller.
A spokeswoman for the Scottish Government said: "It is vital that patients continue to have confidence in the service's ability to respond to emergencies and as such we ask the service to consider these findings to ensure any lessons can be learnt."