FatalAce and Aris are not wrong. Somehow when you try to intentionally force yourself to go faster you will end up slower, because you rush through the flourish, you grip the cards too tight, your wrist tenses up and all that. When you stop doing that flourish for say, a week or two, it ends up like a mini cardistry sabbatical. You already know the move, so when you come back after a week or two you don't have to jump the hurdle of learning the moves, but your body has that understanding that it knows the moves yet hasn't done it for some time. So naturally, it assumes that its a new move (with old experience), and you will find yourself MUCH faster.
The key to a good sybil, IMO, comes from three things. Firstly, wrist action and how consistently far apart you can keep your hands from each other. Sybil is a two packet card that straddles a packet across the index finger of one hand and the thumb of the other. Try to keep your hands the same distance apart from one another throughout the whole flourish; this helps with the squaring of the packets, and when there's a bit more tension on the packets it becomes easier to move around. That being said, you need to move your wrists to get your Sybil fast. Many people attempt to do a Sybil with little wrist movement but a lot of finger movement, but hey, we're not Greg Irwin. When you move the wrists, and get used to the flow of moving the wrists, you can use your wrist strength to improve your speed. Wrist-propelled speed is way faster than finger-flexibility-propelled speed. Secondly, muscle memory. There's two parts of the Sybil that contribute greatly to speed. One is the part where you switch the packet from your ring finger to your second finger, and the other is when you are picking up the packet with your fourth finger and last finger. The former is where I slowed down when practicing, but I eventually got used to moving my fingers fast enough. The latter is a bit more of finger dexterity; you need to get used to really gripping the packet at the first touch so that you can move on quickly without risking dropping the packet. The third and final way to get a good Sybil is actually a having a good rhythm. Sometimes speed can only come from a good rhythm that allows you to gradually accelerate. Wanting speed but not rhythm or flow in a flourish is like asking a Airbus A380 to start flying without taxi or runway sequence. For example, look at D+M, notice that his sybils all have a certain, distinct rhythm to it. For me that worked, though I am not sure about you.
