DIV Invisible - Fade In / Out Too Slow

I have a div that is invisible all the time, and when one popup is activated, that div will fade in to darken the website background, like this:
And I'm using the following code when the darkness appears:

PHP Code:
function raiseOpacity(obj,counter) {
obj.style.opacity = counter/100;
counter +=2; //Raise 2% each time
if(counter < 100) setTimeout(function()
{raiseOpacity(obj,counter)},10);
//Repeat the incrementation again}raiseOpacity (document.getelementbyid('example'),0); 
//Just an example of how the function is called.

But it is too slow, the browser hangs alot while doing this. Is there a faster way to do this fade in stuff? I've seen a lot of websites that do this and the browsers is fast, but I couldn't extract the code from them.