I found that the speed is not as important as you think. The ability to count down a deck of cards in your hand doesn't really happen at all at the table. The majority of times at the table you will be canceling pairs and groups. It wont be long before you are fast enough to keep up with even the fastest dealers, more practice and counting starts to become like breathing and you "just do it." Then you can really focus on your act and acting natural at the table instead of staring at the cards looking too focused. Being able to hold conversations and be aware of surroundings around you all while keeping count and playing is something you need to accomplish.