Cold start is a term used in cloud computing to describe a scenario when a service takes time to get ready(spinning up an instance) to serve a request.
It is known issue of Azure functions running under consumption plan .
In consumption plan, the backend infrastructure gets added when load increases and infrastructure is removed when load decreases.
When load is zero, no infrastructure. So when new request comes in, the time taken to get the infrastructure ready to serve it is called cold start time.