[Reg 5.14.2 -> 5.15.0] Significant performance decrease when loading QtWebEngine pages simultaneously in many windows

There is significant decrease in performance when loading QtWebEngine pages simultaneously in many windows.

Testing was done initially by comparing Qt 5.12.12 with Qt 5.15 and Qt 6.2.9 and Qt 6.5.2. Last known good seems to be Qt 5.14.2

Qt 5.12.12 outperforms the latest supported versions when downloading simultaneously multiple heavy webpages. Please find the reproducer attached. The test program uses yahoo finance to load the webpages, please note that traffic can vary when stock trading is open vs closed, increasing the number of opened windows makes the difference more evident, if one cannot reproduce with default.

Additionally if testing in EU, the test breaks due to EU's requirement for using cookie pop ups for data collection approvals etc. so using VPN/proxy to route via US is recommended in order to avoid pop ups.

In general, testing is better to be done in release mode due to:
https://bugreports.qt.io/browse/QTBUG-110504

Way to reproduce:
1) Build and run the attached example in Release mode with unaffected version of Qt (E.g. 5.12.12) and after one has done testing (letting it run couple of times from yellow to green/red should be enough), build and run with affected version of Qt (E.g. 6.5.2)

2) Application opens 20 windows and starts to download webpages, color in side of the window is yellow when downloading is in progress, and turns green once finished unless it downloads longer than 60s (color turns to red) and when subsequent test runs for windows that were green starts (automatically) windows which have been red are turned to purple.

3) Comparing performance between unaffected version (5.12.12) and affected version (6.2.9/6.5.2) it can easily be seen that performance has significantly decreased, with affected versions producing much more "red/purple" windows than unaffected Qt 5.12.12