A freshly installed Windows 7 takes very long to scan for updates. The same happens if a Win7 PC has not yet received the updates of middle of 2016.
The reason is that the original updater is very slow, and struggles to handle the large number of updates. It can even happen that it never completes, because it may need more than 10 hours, and it restarts after a reboot. So if a PC is shut down every evening, the updater never suceeds.
It is possible to speed this up a lot, by doing the following steps, precisely in this order (skip the steps before the servicing stack if you want to speedup an existing Windows installation):
Now let Windows scan for updates (and enable the automatic search). This scan will be very fast. Currently (June 2017) this finds "only" 56 updates. This number of updates can be installed in one run (you should not install more than about 50 updates at a time, because the installer has a memory leak).
The above order really is the correct one, despite of them being not chronological. Reason: The Convenience Rollup cites KB3020369 (Servicing Stack 2016-04) as prerequisite, but that was superseded by KB3177467 in 2016-09.
"Platform Update KB2670838 must be installed before both rollups, otherwise you will need to re-install them after it." (source)