If the main server is slow, use official mirrors (China: mirrors.tuna.tsinghua.edu.cn/qt/ , Germany: ftp.fau.de/qt/ ). Always ensure the URL path ends with /5.15.2/ and the filename contains opensource .
, you can still download pre-built offline installers directly: Qt Account portal Availability: qt 5152 offline installer
Despite being a stable release, the has quirks. If the main server is slow, use official