Sorry about your issues, but also thank you... Because I've seriously been so on the edge about whether to take the free update (my PC Manufacturer says it works fine, and basically every piece of software, the developers say it works fine)... But I just don't trust it. I don't need any Windows 10 features and everything works fine already... I think I'm gonna just stick with 7 forever pretty much. And I have Windows 10 on my two non-music-making laptops and it works great. But I'm just too nervous about random weird issues on my studio PC.

Well, obviously I can't say what's wrong in my case, or even if there actually is something wrong! Maybe I'm just imagining that it's much slower, although I doubt it. But I do have a possible solution for you. It's a bit of work but it gives you an option.
 
You can upgrade to Windows 10, but you have 30 days to go back to your previous version. The good (if you want to look at it this way....) news is: Your license is stored in "the cloud" so you can go back to your previous version but still be able to upgrade back to Windows 10 again in the future without having to buy a copy.
 
That's why I can go back to Windows 7 now but still install 10 at some point if/when I think I/they have the problems figured out. Of course, I can't yet comment on the downgrade process. Hopefully it's not too painful.
 
Scott

Hi, Scott... I was also experiencing slow startups of Sonar projects after upgrading to Win10...Seemed strange after all the improvements to loading in Kingston...  I uninstalled SPlt, using the clean install procedures provided by Scook:   https://www.cakewalk.com/Support/Knowledge-Base/2007013392/SONAR-2015-Clean-Install-Instructions
 
Now my Sonar bootups/projects are blazingly fast.  Faster than ever.  Large track counts (75+) and multiple Kontakt files which use to be slow load in 15 seconds... really worth the SPlt reinstall... I did encounter new problems using the start screen but I've just stopped using it after reporting these problems to the bakers in the fault reporting system.  My experience is reported here:  http://forum.cakewalk.com/Reinstalling-platinum-m3332089.aspx
 
Best of luck... and I'm glad to know the clean install proceedures for win10... might do this during the rainy season to start totally fresh although it'll be a real pain given everything on my boot ssd.