A functional requirement is about what the system shall do. The term "functional" comes from "function" meaning something that takes some input and produces some output.
A non-functional requirement is about how well the system shall do what it does. These are usually quality requirements, such as performance, reliability or ease of use.
Your requirement does not tell at all what the system does, but it tells how the system shall do it. It is hence without any doubt a non-functional requirement.