Hi, and welcome to the Qt Dev Net!

@beahacker said:

I have an bad experiences when I downloaded software from Qt websites. It is slow and unreliable. I failed to download Qt 5.5 alpha and other Qt products several times. Anyway, for your information, I am from Singapore.

It sounds like you were assigned a slow mirror. Choose a different mirror, like @sierdzio suggested.

Do you want Online or Offline installers?

If you want Offline installers or source code, go to the download page, click "View All Downloads", and click on the "(Info)" link next to the package you want.

If the "(Info)" link is missing, manually add the.mirrorlist extension to the end of the download URL, e.g. http://download.qt.io/development_releases/qt/5.5/5.5.0-alpha/single/qt-everywhere-opensource-src-5.5.0-alpha.zip.mirrorlist

If you want to use the Online installer, use the Qt SDK Repo Chooser tool.