Some background information
The issue appeared with the release of Windows 10 in July/August 2015 and affects - as far as I know -
Windows Vista SP2 and Windows 7 SP1.
The Update Agent behaves like this because Microsoft keeps all updates in the update catalog (wsusscn2.cab),
even superseded ones. The Update Agent checks all updates in the catalog if they are applicable/required for
the current system. Updates that are already installed are skipped in the process.
When some update has been replaced/superseded by a newer one, while there's only the superseded (older) update
present on the system, all of the update's predecessors are checked whether they are applicable/required for
the system. And this check of superseded updates takes so long und causes high CPU load, often taking hours
depending on the hardware. As a result the check for new updates takes ages.
Since every month on second Tuesday new updates are published, the issue reappears every month, and almost
every time a new set of updates is required to speed up the check for new updates.
By the way, the internet connection and Microsoft's servers are not relevant for the search for new updates,
as has been assumed a lot of times all over the internet. A look at the Windows Task Manager reveals that
there's a high CPU load (100% on one core), and the memory usage is quite high as well. Network activity is
hardly to see, except at the beginning of the scan.
A more detailed explanation - maybe even better than mine - can be found at