I tried a simple benchmark of new QVariant in Qt6 and it shows significant slowdown (2 to 3 times) when compared to Qt5.

QElapsedTimer t;
t.start();
int sum = 0;
for (int i = 0; i < 100000000; ++i)
{
    int a = 1;
    QVariant v = a;
    int b = v.toInt();
    sum += b;
}
qDebug() << sum << t.elapsed();

On my environment (Windows 10, MSVC 2019, 64bit) the measured times are around 1.5 secs with Qt 6.0.1 and 0.6 secs with Qt 5.15.2. This is 2.5x slowdown.

I measured similar slowdowns with QString.

I know that these numbers quite insignificant per se and conversions are still fast in most real life situations, but this can mean that QVariant usage becomes a burden in some performance heavy scenarios. Natural example which comes to my mind is sorting of a large data model which require conversions to/from QVariant of each value.

But maybe there is potential for some type specialization for most prevalent types such as int, double or QString which could regain some more performance.