One commonality between most, if not all, FaaS providers is the issue of cold starts. Cold starts are defined as the behavior where an invocation of a cloud function which has not been executed in a while takes a considerable amount of time to initialize before fulfilling the request. If you have used Docker, for example, you'll know that creating a new container from an existing image takes slightly longer than starting up a container you have previously run. This Docker container behavior is analogous to the way cloud functions, whether it be AWS Lambda, Google Cloud Functions, or Azure Cloud Functions, behave. If you do any searching around the internet for serverless cold starts, you'll find several blog posts and documentation on the matter.
There isn't a silver bullet for bypassing the cold start issue. However, there are several things to be aware of so that you can minimize their impact on your application.