RTT Client hint request header field provides the approximate round trip time on the application layer, in milliseconds. The RTT hint, unlike transport layer RTT, includes server processing time.
The RTT value is rounded to the nearest 25 milliseconds to prevent fingerprinting; There are many other mechanisms an attacker might use to obtain similar round-trip information.
The hint allows a server to choose what information is sent based on the network responsiveness/latency. For example, it might choose to send fewer resources.
Vary header is used in responses to indicate that a different resource is sent for every different value of the header (see HTTP Caching > Varying responses). Even if
RTT is used to configure what resources are sent consider omitting it in the
Vary header — it is likely to change often, which effectively makes the resource uncacheable.