@George Aggrey Premium Plan tackles cold starts by having at-least one instance active and then at-least one-prewarmed (until the max scale-out limit) which are instances that have already been started up.
So, after being inactive for a while, the number of instances go down and if there is a sudden surge in requests, more than what the warmed instances can handle could cause performance issues while the app is scaled out to the required amount.
But since the number of ready and pre-warmed instances can be configured, you will just have to configure them accordingly.