Since we are talking performance here, I'd suggest using

const auto it = d->entries.constFind(guid);
if (it != d->entries.cend()) {
    ....
}

This way you avoid looking up the the entry in the hashtable twice (once in contains() and once in operator[]).

This change does not seem related, could you explain it a bit more?

Indeed, but note that this is the dummy feed storage, that is almost unused (it is used if you disable archiving, or if the other backends fail to load)

Sadly it is. The markImmediatelyAsRead flag is described in the user interface as «marking feeds as read when new articles arrive».
The current implementation does that, but also marks every article as read at startup.
This does not map to the current user interface description, and implies reading every article for such feeds, seriously impacting startup time.

Perhaps you can merge this two constructors no ?

coding style. Remove space after &