Performance degradation with Qt6

[PierreRaybaut]

This is the same problem as in this PlotPy issue. Actually, this should have been the other way around...

The qwt\tests\test_loadtest.py may be used to reproduce this issue. For example, on Windows 11 with an average machine (Intel Core i5-1135G7):

• [Python 3.11.3, Qt 5.15.2, PyQt v5.15.11 on Windows] Average elapsed time: 491 ms
• [Python 3.11.3, Qt 6.8.1, PyQt v6.8.0 on Windows] Average elapsed time: 821 ms (+70%)
• [Python 3.11.3, Qt 6.8.1, PySide v6.8.1 on Windows] Average elapsed time: 1007 ms (+100%)