When migrating from Qt 5.15 to 6.0 we encountered a severe performance decrease in QFile::copy. While copying large files with QFile took about the same time as the command line cp, the Qt 6.0.1 version takes more than 10 times to finish its work.

Here are the results from a test program you can find attached:

(Test.aaf is a 1,3 GB zip file.)

$ rm test.aaf
$ ./copy input.aaf test.aaf
Qt: 6.1
copying input.aaf to test.aaf
copying took 8229 ms

$ rm test.aaf
$ ./copy input.aaf test.aaf
Qt: 5.15
copying input.aaf to test.aaf
copying took 656 ms

For comparison, cp on the command line:

$ time cp input.aaf test.aaf

real 0m0,697s
user 0m0,003s
sys 0m0,690s

Is there any change to speed up the QFile::copy method?