On some Windows 7 (SP1) computers, Windows Update becomes super slow even the computer is reasonably fast. There's one solution that seemed to have worked well for me (after trying a few other suggested solutions).
Sometimes the Windows Update feature basically hung and you could be staring the "Checking for updates..." screen for days without it finishing - if you let it run that long! Based on my research online, it seems to be a peculiar bug that has root in both the service itself as well as Windows kernel itself. It surfaced a couple times in the past without Microsoft explaining too much about it. This is an article about the latest round (since May 2016).
What solved it for me was KB3172605. There are two things you may also need to keep in mind.
It needs KB3020369, so install that first if you don't have it on the system. Installed updates can be found in the "View update history" link from the Windows Update screen.
The stand-alone installer may not work at all if you simple run them, basically suffering from the same "Checking for updates" bug. To get out of that catch-22, stop Windows Updates service first before running the installers.
Once I installed the hotfix (I already had the prerequisite) and restarted the computer, Windows Updates returned results in a very speedy 8 minutes. It's truly day and night! For about 3 months I couldn't get any updates, so it's a huge relief to be back in the good.