WebEngine slow to loads some websites depending on GPU config and window size

With some QWebEngineView sizes some webpages are extremely slow to load on platforms without GPU.

Symptoms

Some pages take a very long time to loads.

For instance on https://twitter.com/explore the page itself will load quickly, but then the loading of the actual content will take a long time (10+ seconds some times).
During this time the "busy indicator" will be animated, but the page will have a very slow reaction time to user interaction. Notably cursor changes e.g. change from arrow to hand when hovering a button has a latency of several seconds.

The the dev tools networks consoles some request are queued and/or stalled for multiple seconds.

How to reproduce

Qt must be running with OpenGL desktop.
Chromium must not be using OpenGL (either on a PC with hardware acceleration or by using --disable-gpu.

The viewport must be "just right ". I was able to reproduce with a width of 1705 and height of 600, it is not the only one but the issue seems to be very dependant on the geometry.

Mitigations

On PC without GPU, either setting
1. QTWEBENGINE_CHROMIUM_FLAGS="--ignore-gpu-blocklist --disable-webgl --disable-webgl2 --disable-accelerated-2d-canvas" (basically leaving only accelerated compositing and rasterization is enough).
2. or QT_QUICK_BACKEND=software
seems to fix the issue.

Notes

I have not tested with WebEngineView