From Christof Donat:

JQuery could check if context.getElementById exists and if it doesn't walk the elements in the context and return the first Element it finds that has the given ID:

if( context.getElementsById ) elems = [context.getElementsById(id)];
else {
	var f = function(i,c) {
		if( c.id == i ) return [c];
		for( ch in c.childNodes ) {
			if( ch.nodeType != 1 ) continue;
			var r = f(i.ch);
			if( r.length > 0 ) return r;
		}
		return [];
	}
	elems = f(id,context);
}

That means that for browsers with a good implementation of getElementById()

$('#myid',context) is much slower than $('#myid'), but it returns the

expected result. At least that should not be slower than

$('.myclass',context).