Maybe your Mac was in fine fettle before you upgraded?
The irony of my situation was that I did lots of research before I 'upgraded', as I wanted to continue with Aperture (I am a professional event photographer with over 2 million images in around 100 libraries). So, I waited until a few fellow Aperture users had gone to El Capitan and said Aperture still worked. I also did a lot of Googling and found a few reports of people with older MacBook Pros, with low RAM, finding El Capitan actually sped their machines up...
However, i has killed my MacBook Pro and I can't use it 😟 Right now my main Aperture library is 'Repairing' for the 3rd time this week (and it's only Tuesday)... The repair is 30% complete after 2 hours 😟
Luckily, I kept my iMac at Yosemite and at least I can do my work when I'm in the studio, but I have a funny feeling my laptop won't be another Apple product, when I can stand this no longer...