TL;DR: There's only one change to previous month due to April Patchday: KB3145739 replaces KB3139852.

Validity/Coverage

Valid as of: Apr 2016 (2016-04-12)
Valid until (presumably): May 2016 (2016-05-10)

This HowTo is valid for Win7 SP1 and Vista SP2, which have been newly installed or haven't been updated for some time. It should also help on systems that were up-to-date last month - just install the missing updates from the list below in this case.

These instructions were only tested on Win7 SP1. However, the list of updates should speed up the search for updates on Vista SP2, too.

Solution to the issue

Install the following updates BEFORE letting Windows search for updates, to avoid this very search taking "forever":

KB	Win7 x64	Win7 x86	Vista x64	Vista x86
KB3145739	Download	Download	Download	Download
KB3078601, "loyal" companion since August 2015	Download	Download	Download	Download
KB3087039	Download	Download	Download	Download
KB3109094	Download	Download	Download	Download

On newly installed Win7 SP1 systems it's not really necessary to install the updates from the list above; it's sufficient to install any of the current Windows Update Clients (December 2015 or later) from the table below. But since the updates from the list will be installed automatically anyway, it doesn't matter if they're installed manually beforehand.

To reduce memory and CPU usage while searching for updates, the following updates should be installed as well:

Windows Update Client
It's sufficient to install one of these packages:

KB		Win7 x64	Win7 x86	Win8.1 x64	Win8.1 x86
KB3075851	August 2015	Download	Download	-	-
KB3083324	September 2015	Download	Download	-	-
KB3083710	October 2015	Download	Download	-	-
KB3112343	December 2015	Download	Download	-	-
KB3135445	February 2016	Download	Download	-	-
KB3138612	March 2016	Download	Download	-	-
KB3138615	March 2016	-	-	Download	Download

Every one of them reduces the Update Agent's memory usage (having a good effect, especially on systems with less RAM).

The following update is (most likely) not required if the Windows Update Client of December 2015 or later is already installed:
KB3102810, reduces Update Agent's CPU load.

Steps to take after Windows installation

Disable Automatic Updates:
Start > Control Panel > System and Security > Windows Update > Change settings > "Never check for updates"
(Optional) Stop "Windows Update" service: Open Windows Task Manager > Tab "Services" > Right-click on "wuauserv" > Stop service
Download the updates mentioned above and install them via WUSA on a CMD shell (as Administrator):
```
"%SystemRoot%\system32\wusa.exe" "C:\full\path\to\Update.msu" /quiet /norestart
```
Tip: Use the TAB key to let the system complete the directory and file names, to avoid typing errors.
Reboot the system.
Now let Windows search for updates, which should be finished in less than 15 minutes.
Re-enable automatic updates (see step to disable them, but select one of the options to check for updates automatically now), and install the remaining updates as usual.

Misc

Thanks to T. Wittrock, author of WSUS Offline Update, Denniss and the community.

Regards
Dalai