In my web application, one of the component is slower than other components, how to deal with that?

Suppose I have a micro-services based architecture. Some of the micro-services are more heavily loaded than others.How I decide how much scaling I do for each of the components? My understanding is I do load testing & find out how much load each of the service can bear & based on that some service I scale say by a factor of 3, some by factor of 5 etc. Now suppose I have a service - payment service - which depends on 3rd party payment gateway which is slow and not in my control - how to deal with that? I am free to containerize the application & user docker/kubernetes.