QSet subtract function is slow

Q_INLINE_TEMPLATE QSet<T> &QSet<T>::subtract(const QSet<T> &other)
{
    QSet<T> copy1(*this);
    QSet<T> copy2(other);
    typename QSet<T>::const_iterator i = copy1.constEnd();
    while (i != copy1.constBegin()) {
        --i;
        if (copy2.contains(*i))
            remove(*i);
    }
    return *this;
}

Q_INLINE_TEMPLATE QSet<T> &QSet<T>::subtract(const QSet<T> &other)
{
    if(&other == this)
    {
         this.clear();
    }
    else
    {
         for(QSet<T>::iterator it = other.begin(); it != other.end(); ++it)
         {
                this.remove(*it);
         }
    }
    return *this;
}