Page Speed recommends:

Use server-side storage for most of the cookie payload

Store only a unique identifier in the cookie, and key the ID to data stored at the server end. You can use server-side cookies for both session and persistent cookies by specifying the expiry date/time on the cookie.

Remove unused or duplicated cookie fields

The fields set by a cookie at the top-level path of a domain (i.e. /) are inherited by the resources served off all paths below that domain. Therefore, if you are serving different applications on different URL paths, and you have a field that applies globally to all applications on a domain - for example, a user's language preference - include that field in the cookie set at the top-level domain; don't duplicate the field in cookies set for subpaths. Conversely, if a field only applies to an application served from a subpath - for example, a UI setting - don't include that field in the top-level cookie and force the unused data to be passed needlessly for other applications.

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