All of the following methods do not modify the input string, and neither do they construct a new string from any concatenations that would justify the allocation of a new QString:

src/transaction.h:

    /**
     * Returns the package name from the \p packageID
     */
    static QString packageName(const QString &packageID);

    /**
     * Returns the package version from the \p packageID
     */
    static QString packageVersion(const QString &packageID);

    /**
     * Returns the package arch from the \p packageID
     */
    static QString packageArch(const QString &packageID);

    /**
     * Returns the package data from the \p packageID
     */
    static QString packageData(const QString &packageID);

In KDE/Plasma Discover there is a copy-pasta of packageName method for the sake of performance improvement. Here's a copy of that comment of top:

// Copy of Transaction::packageName that doesn't create a copy but just pass a reference
// It's an optimisation as there's a bunch of allocations that happen from packageName
// Having packageName return a QStringRef or a QStringView would fix this issue.
// TODO Qt 6: Have packageName and similars return a QStringView

Changing it in packagekit-qt now would be an ABI brakage, and you can't overload by return types in C++. But it should at least still be mostly API-compatible, as QStringView implicitly converts to QString when needed.

Alternatively, we could overloaded counterparts for all those methods which both take and return a string view. That would require explicit porting in apps, if they want to take advantage of performance gains.