Some new issues with Windows 10. 2 of my laptops were not able to fast shutdown and 1 of them was getting a low Windows memory error even though it was only using Firefox and had 4 GB of RAM. Due to this low memory issue which under feedback I see other users reported, I would suggest you don't upgrade to Windows 10 if you have less than 4 GB of RAM. Instead upgrade to at least 4 GB for a notebook and if a desktop, you should be able to easily upgrade to 8 GB.
To solve the low memory issue, I did the following. I went into processes to see what was taking up so much memory. 1 of the problems I saw was Cortana. I can see why you might want to use it on a Window's phone, but I never wanted to use it on my computers, but it sits in RAM of at least 30 MB and is always on. I was able to get rid of it. I then saw my current antivirus program was taking up way too much memory so after uninstalling it, I searched for and installed a low memory anti-virus application. I also made changes to the virtual memory of Windows. I also choose Chrome instead of Firefox for the laptops. I was also able to get rid of some other junkware in memory. The new Edge Browser also looks good for those that want to use it. For my 8 GB desktop, I decided to use WaterFox which is a 64 bit version of Firefox. The desktop was not getting a low memory error but I wanted a faster browser. Finally, I decided to use LibreOffice instead of Microsoft Office since unlike Microsoft Office, it is free to use and works well and fast on Windows 10.
Note the new November threshold 2 patches are out for Windows 10, but they may not automatically install due to the fact that your computer is given 30 days to roll back to Windows 7. However, you can manually install them, and they do make the computer more stable. After installing these patches, your computer will boot faster into Windows 10. However, Microsoft made a big mistake especially for older computers. Under change what power buttons due, "Turn on fast startup" is checked and greyed out. Instead of making the computers start faster it makes the computer slow to shut down or even not shut down. First make sure that under power button it says when I press the power button Windows shuts down. Then click change settings that are currently greyed out. Next, uncheck turn on fast startup. Some of these major patches may revert settings you previously set back to default so you should check again that all settings are how you want them.