pacman -Syuu : error: failed retrieving file ("Operation too slow" and "SSL certificate problem")

You can disable the timeout of pacman with the parameter --disable-download-timeout. Then you should be able to download

http://repo.msys2.org/mingw/x86_64/mingw-w64-x86_64-qt5-5.12.1-5-any.pkg.tar.xz

It's described on the man-page for pacman.

Also --gpgdir to Specify a directory of files used by GnuPG to verify package signatures might be useful.

Additional you can consider using the parameter --noconfirm to bypass any and all “Are you sure?” messages.

About the certificates I'm not sure, perhaps the local paths are not configured correctly. Nevertheless you've two domains, that still serve by http.

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In browser the download works btw. it just has finished, the screenshot is still showing the download in progress: