Case 1:
for(var i=0;i<10000;i++){
$("#div"+i).toggleClass("selected",true);
}
Case 2:
for(var i=0;i<10000;i++){
if($("#div"+i).is("selected"))
$("#div"+i).toggleClass("selected",true);
}
Case 3:
divs.find(".selected").toggleClass("selected",true);
In the following test I create ten-thousand divs. Every fifth div has a 'class="selected"' attribute. I am running three tests above on all the classes.