There is so much involved in octave playing (fingers, wrists, arms). I thought I had octaves down--I practiced them every day (scales, arpeggios, jumps, repeated notes, figures from repertoire), but I've lately come to the realization that although I could play fast I wasn't playing with control. There are different principles I'm practicing with now, and I know practice much slower. The first is to make sure your fingers are relaxed--so when you first go to practice, play your first octave and lift your hand, letting your fingers come to a rest and especially feeling them relax. Then try to play multiple notes and do the same thing--with this I'm trying to set a baseline of relaxation--this comes to be more important when you play legato octaves (with the fifth, fourth, and sometimes third fingers involved).
The next thing I focus on are my wrists. I've had the habit of keeping them completely rigid and just using my arms--I could do this fast, but inconsistencies would emerge if I had to play extended passages, and there is a harsher tone in which I couldn't get the nuances I wanted. So I try to focus on keeping my wrists loose. I often do this with exaggeration, having almost floppy wrists (like a kind of effortless basketball dribbling going mostly with gravity on the way down without my wrist trying to leverage extra force and then relaxing completely as the arm rebounds), just to make sure my wrists are relaxed. As you get more comfortable a flexible relaxed wrist will help you shape tones better and connect tones (having a tensed, fixed wrist translates into tension for the entire mechanism--hand, wrist, arm, shoulders).