If the fixes for Windows Update listed in the other answers have not
fixed your problem, the slow updating might be caused by the way that Windows
Update works on the affected computers.
In another answer of mine, I have explained
that Windows Update constructs in memory a tree of all the updates which could
apply to a computer, then prunes it with a view to the updates already installed
on the computer, to finally arrive at the set of updates that need to be installed and the order of installation.
The time taken for this brute-force process is a function of the total number
of updates available for this platform, since the last service pack.
Every service pack sort of defines a new platform for which updates start
to accumulate anew. Also, Windows Update need to transfer large amounts of data
from the Microsoft servers who may be overburdened.
Windows 7 SP1 came out on February 22, 2011, more than 5 years ago, and since then
the number of updates has grown enormously. In addition, Microsoft's
Windows Update servers now give priority on bandwidth to Windows 10 clients.
So all in all, Windows 7 SP1 is left to suffer.
To solve this problem, Microsoft has lately released the
convenience rollup for Windows 7 SP1, which, exactly the same as a
service pack, serves as a starting platform for updates.
Installing it results in a much smaller
updates tree which is much faster to download and process,
since only updates posterior to it are considered rather than all updates since 2011.
Unfortunately, it is not available via Windows Update and has to be
downloaded and installed manually.
For more information on the convenience rollup for Windows 7 SP1, read
Microsoft overhauls Windows 7 and 8.1 updating -- but don't call it a service pack.
This convenience rollup is the only way by which one can reduce the
Windows Update running time on Windows 7 SP1. Another one would be to
launch it on hours in which the Windows Update servers of Microsoft
have more available bandwidth (early morning or late night).
Admittedly, the problems you observe are a bit extreme, and might be related
to some inefficiency that relates to the particular setup of these computers.
I think that some combination of factors has enormously increased the time that
Windows Update takes to download and prune its update tree.
This might even be a Microsoft bug.
You might gain more information on the problem by watching Windows Update
while it is working, as regarding memory usage, network activity and disk access.
Installing the convenience rollup for Windows 7 SP1 is the only way I can think
of to cut this Gordian Knot of Windows Update on the affected computers.
However, it will probably stop working some time in the future, so has to be applied