I refurbish computers for a living and have been battling slow updates for months now. Two identical machines, identical hardware and software, start at the same time. Machine 1 gets its updates in a few hours, machine 2 takes days... There is no rhyme or reason as far as I can tell. There are some downloads that one can acquire from Microsoft to manually install certain updates. People claim that the right updates in the right order will solve the "slow update" problem, and I have had mixed results with all of them. What I do know is that the updates that appear to be causing the problem are related to Windows 10 app compatibility with non-Windows 10 operating systems.
Manual updates that I have found so far:
From a clean install---
Windows6.1-KB3020369-x64 = Stack Servicing Update April 2015 (required to search for updates)
windows6.1-kb3125574-v4-x64_2dafb1d203c8964239af3048b5dd4b1264cd93b9 = Conveniance Rollup April 2016 (approximately 480MB worth of "really important" updates
Windows6.1-KB3168965-x64 = MS16-090 Security Update Kernel Drivers July 2016 (this was a reported fix to earlier kernel updates in Q1 and Q2 2016 that had issues on some machines)
From an install slightly out of date (say two months)
Windows6.1-KB3138612-x64 = Windows Update Client March 2016 (another required update to receive updates, much like KB3020369 above)
Windows6.1-KB3145739-x64 = MS16-039 Security Update Risk Graphical Vulnerability April 2016 (reported to work with KB3138612 in one solution to fix the slow updates)
Also helped in some scenarios---
Windows6.1-KB947821-v34-x64 = System Update Readiness Tool October 2014 (as far as I can tell, this is the update that a fresh install requires in order to see anything past October 2014 in the way of updates)
I am using 64 -bit operating systems, hence the "x64" at the end of the KB number. These should all have an x86 version for 32-bit operating systems as well, iknow for a fact most do I just have not tried all of them on x86. And as far as I can tell, they are not version specific to Windows 7. Meaning that they work for Home, Professional, and Ultimate the same.
Again, I have had mixed results using identical hardware and software. Worst case scenario, it has dropped my search time from literally weeks, to hours, sometimes over night. One machine literally took two weeks. I did a clean install August 15, 2016, and yesterday, August 29,2016, It finally had its updates. Other cases, updates were running over night with no success, I restarted and applied the manual downloads, and then updates were found within two hours.
I am still looking for a solution that will work all the time. In fact, I am still battling this problem on many devices. I hope you have better luck. And that this Windows 10 app compatibility nonsense gets resolved soon.