There are probably other targeting factors that can be applied before loading the mixins, but geolocation seems like the biggest one that would let us avoid loading unneeded modules for a lot of readers.
Right now it seems like if a campaign that requires special mixins is running somewhere, all users of that wiki load that CN mixin. Since the information about mixins seems to come from choiceData, where Geo targeting is also defined, it seems like the decision to load the mixin or not could depend on the geotargeting, to avoid loading mixin modules that won't be needed by the current user.
Right now, browsing enwiki from France, I see that there are two campaigns requesting the largeBannerLimit mixin:
{F17565397, size=full}
Both of which are targeting the Netherlands, where I'm not.
Yet, I'm loading the largeBannerLimit mixin:
{F17565412, size=full}
I don't know if mixins can affect the targeting logic. If they don't, they should only be loaded after all targeting logic has run, to make sure that extra JS modules are only loaded for users that need them.