According to documentation.
RUN_TIME : The amount of time that the request took in milliseconds. Requests with a value over five seconds are considered long-running requests for the purposes of the Concurrent Long-Running Apex Limit.
But we also have CPU_TIME which has 10 seconds and 60 seconds timeout depending on Sync and Async transaction.
Say we have 10 concurrent requests running in Apex (SOQL may take more time, but as SOQL is not counted against CPU TIME we should be good), which is still under CPU_TIME limit, and a 11th request comes. Why does SF throws
long running concurrent requests exception.
This means technically when the load is heavy the total execution time should be less than 5 seconds, even though CPU time is 10/60 seconds.