I really, really wish i had never updated from that. It was faster, smoother, and more stable there than it was before and after. Definitely way better than Mavericks. I push this thing hard with premiere/ableton live/etc too.
Also, I absolutely despise the El Cap design. The flat look control widgets and icons, the barely visible grey text, the secret hidden controls that only appear when you mouse over them, it all makes me cringe. (Yes, Mav has a few hidden controls as well, but at least now I know where they all are. If I upgrade, I have to go on the secret control easter egg hunt all over again. Life is too short for that kind of bullshit.)
Sure, but most 2FA happens over SMS and that is already insecure. Updating your OS won't help if someone has hacked into the SS7 network.