The minimum setInterval() duration in JavaScript is 4 ms. Although the minimum value you can set is 1 ms the code is actually executed in 4 ms since this is subject to a timer is subject to an algorithm specified is the HTML5 spec. You can click on the link to learn more.
You can try counting by 2 or 3 so it appears to go faster. You can even randomly change the increment to a number between (for example) 1 to 5. Then each time it counts it will add a random number within that range.