To be completely honest, I wouldn't worry about security things on OS X, unless I hear about something nasty, which is pretty rare for OS X and iOS. If Mavericks makes the best out of your computer, then by all means go back to it. I'd rather use my hardware at its best than in a bogged down state that Apple loves to do recently with OS X and iOS. In fact, I have a rig that I just got running the other day with Mavericks on it. Aside from the need of a graphics card this thing is buttery smooth, and supports a ton of things still. The only setback Mavericks has is the fact of the gorgeous Aqua UI. Much like the bottleneck with iOS 6, Many developers won't put their apps on it because of the UI, MS Office 2016 is a good example of this. I was fine with Apple's way of updates, but when Jony Ive managed to completely screw up the UI and experience, I couldn't stand it because they forced this new UI on us. I was cool with it for a week, but then I began to hate it, and that hatred only grew.