Scalability As A Functional Or Non Functional Requirement

I am currently tasked with writing Software Requirements Specification (SRS) document for a project. Effective sharding (based on specific criterion) and Scalability are key requirements of the project.

Scalability is traditionally classified as a non-functional requirement. My question to the community is that if scalability is crucial to a project, would it still be classified as a non-functional requirement? Are their cases when scalability requirements would be best classified as functional requirements?