You could use a struct for both, e.g.:

struct SpeedSample {
    KIO::filesize_t remainder;
    qint64 elapsedTime;
}
QVector<SpeedSample> transferSpeed;

This would emphasize the two values always come in pairs, and would even remove some code above.

.. at most ...

Strange way of writing {0, 0} ...

The result is the same in this case - after 7 seconds, the initial items are shifted out of the array. During these 7 seconds, the first element is always {0, 0}. It does not matter if last() is the 2nd or 7th element.

That wasn't there in the old code because it has this guard:

const qint64 diff = currentTime - d->start_time;

(effectively calculating elapsed)

Then

if (diff - d->last_time >= 900) {

So we known first-last is non-zero

Should the new code be

https://phabricator.kde.org/P287 ?

It seemed to work for me.

Indeed that should be the code here.
The old code checked the elapsed time since last "calcSpeed()" call whereas the new code checks the elapsed time since the timer started.

I doubt it will be zero here. elapsed_time stores elapsed time since the timer started so it will continue to grow (?) and the difference will always be > 0.

This results in a fast ramp up when the transfer has stalled for more than ~8 seconds, like at the begin of a transfer. Likely needs a comment.

std::array can not be resized. At first, there is only one element.

This requirement could be avoided around by filling the the array with {0,0} elements, which results in identical behavior.

We would also need a method for shifting the entries. (.removeFirst(), append()).

Initial item is set to {0,0} before calcSpeed is called.