Currently we're making ~35k req/s from MW to session storage (redis at the point of writing). This is a lot of requests and it's unclear if all of those are required.
One clear source of excess traffic is construction of the SessionBackend: as soon as it's constructed we [[ https://github.com/wikimedia/mediawiki/blob/master/includes/session/SessionBackend.php#L155 | make an attempt ]]to check for session data in the store. This happens for entirely new sessions too, so the vast majority of requests return a 404.
When an entirely new session is created, we can skip doing this read because we can be quite sure the read will result in a 404.