Windows Update Slow on Vista
Windows Update is slow because Windows needs to check if the updates are required or already replaced. Those calls are slow. For Windows 7 Microsoft fixed it with the July 2016 update rollup, but this was never backported to Vista.
To get updates faster for Vista, you should use WSUSoffline. Start WSUSoffline (UpdateGenerator.exe), click on Legacy tab and select here Vista/Server 2008 for the correct CPU architecture (x86 or x64).