Pass notifications between decoupled design layers

I am upgrading a design where data was lightly coupled with the UI:

class Object {
    UI * ui;
};


class UI {
    Object * object;
};

It was fairly straightforward to push update notifications to the ui through the UI pointer, but new requirements for data to be entirely separated from UI and also for different objects to have multiple different UI representations, so a single UI pointer no longer does it nor is allowed to be part of the data layer whatsoever.

It is not possible to use something like QObject and signals due to its overhead because of the high object count (in the range of hundreds of millions) and QObject is several times larger than the biggest object in the hierarchy. For the UI part it doesn't matter that much, because only a portion of the objects are visible at a time.

I implemented a UI registry, which uses a multihash to store all UIs using the Object * as a key in order to be able to get the UI(s) for a given object and send notifications, but the lookup and the registration and deregistration of UIs presents a significant overhead given the high object count.

So I was wondering if there is some design pattern to send notifications between decoupled layers with less overhead?