HTTP2 downloads can be very slow

First off: the numbers here come from testing on a 400MBit/s fiber line, but the actual slow-down is independent of the Internet speed.

Please see the attached test program that uses QNAM to download a ~60MB file from a specific HTTP server. With no special config, the download in 6.2.4, 6.3.1 and 6.4.0 is very slow at <200 KB/s.

Setting QNetworkRequest::Http2AllowedAttribute to false speeds the download up to where it was before with Qt 5: roughly 4-5MB/s.

Just build it, run it and watch the output. You can just uncomment the setAttribute line to test without HTTP2.

I could only reproduce this problem with this specific server at ldraw.org. Please be gentle when testing and don't hammer their server like crazy: this is a non-profit organization run by volunteers

For comparison curl in http2 mode is downloading at around 5-8MB/s:

curl -o /dev/null --http2 'https://www.ldraw.org/library/updates/complete.zip'

 
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 63.6M  100 63.6M    0     0  5651k      0  0:00:11  0:00:11 --:--:-- 6075k