New figures reveal that 4350 critical patients waited more than 20 minutes for paramedics this year, which is more than double the eight-minute target for the most serious calls.
Waiting times for the most serious emergencies have nearly doubled since last year when just 2891 people endured 20 minute waits for vital treatment.
Paramedics took 27 minutes to reach Moray teenager Keiran McKandie, who died after his mountain bike collided with a car earlier this year.
An ambulance service insider told The Scotsman that crews were being diverted from other areas to help cover struggling services in cities such as Edinburgh and Glasgow.
The experienced paramedic said: “It impacts on patients as in lots of jobs you only have a small window of opportunity to get in there and sort people out.
“There are just not enough crews and resources to cover the jobs that are coming in. Often there are people who know the right things to say so you get crews sent to drunks or to Mrs Smith who has lost her phone charger.
“The pressure staff are under is unbelievable. If you have to rely on outside crews to cover the big cities then it shows that places like Edinburgh and Glasgow don’t have the resources.”
Liberal Democrat health spokesman Alex Cole-Hamilton said the “hugely concerning” figures laid bare the rising pressure on the service as the longest waits have increased year on year since 2013.
The number of patients facing long waits in Glasgow nearly trebled from 80 in 2014/15 to 233 this year, while in Edinburgh the number almost doubled from 58 patients in 2014/15 to 108 in 2015/16.
Mr Cole Hamilton, who obtained the figures, said: “Ministers have been warned repeatedly by doctors that paramedics are overstretched and under resourced. These figures show that these concerns are far from scaremongering.”
Ministers recently pledged to train 1000 new paramedics over the next five years to build a more resilient workforce.
A Scottish Ambulance Service spokesman acknowledged that crews were “busier than ever” but insisted that the average response time was around 7.5 minutes.
The spokesman said: “The ongoing development of clinical skills is reflected in the consistently high survival rates that are now being achieved in Scotland as more lives are saved by ambulance teams every year.
“Response times are be affected by several factors, such as sudden surges in 999 demand and requests for hospital transfers, as well as changing weather conditions and turnaround times at hospitals.”