JQUERY + IE = Efeitos+ Lag

Pode ser alguma das coisas que você disse, e pode ser tudo oque você disse tb.

Poste o script para rodarmos, só assim teremos como analizar onde é que está perdendo performance.

//CONTENT WITH AJAX

$(document).ready(function(){
	$(".bg").hide();
	$(".bg").fadeIn("slow");

	checkURL();	//check if the URL has a reference to a page and load it

	$('ul li a').click(function (e){	//traverse through all our navigation links..

 checkURL(this.hash);	//.. and assign them a new onclick event, using their own hash as a parameter (#page1 for example)

	});

	setInterval("checkURL()",250);	//check for a change in the URL every 250 ms to detect if the history buttons have been used

});

var lasturl="";	//here we store the current URL hash

function checkURL(hash)

{
if(!hash) hash=window.location.hash; //if no parameter is provided, use the hash value from the current address

if(hash != lasturl) // if the hash value has changed
{

 lasturl=hash;	//update the current hash
 loadPage(hash);	// and load the new page
	}
}

function loadPage(url)	//the function that loads pages via AJAX

{

	url=url.replace('#','');
	$('#loading').css('visibility','visible');	//show the rotating gif animation

	$.ajax({	//create an ajax request to load_page.php
 type: "POST",
 url: "scripts/load_page.php",
 data: 'page='+url,	//with the page number as a parameter
 dataType: "html",	//expect html to be returned
 success: function(msg){

 if(parseInt(msg)!=0)	//if no errors
 {
 $('#pageContent').fadeIn("slow");
 $('#pageContent').html(msg);	//load the returned html into pageContet
 $('#loading').css('visibility','hidden');	//and hide the rotating gif
 }
 }

	});
}

	$(document).ready(function() {

 // Preload all rollovers
 $("#nav img").each(function() {
 // Set the original src
 rollsrc = $(this).attr("src");
 rollON = rollsrc.replace(/.png$/ig,"_on.png");
 $("<img>").attr("src", rollON);
 });

 // Navigation rollovers
 $("#nav a").mouseover(function(){
 imgsrc = $(this).children("img").attr("src");
 matches = imgsrc.match(/_on/);

 // don't do the rollover if state is already ON
 if (!matches) {
 imgsrcON = imgsrc.replace(/.png$/ig,"_on.png"); // strip off extension
 $(this).children("img").attr("src", imgsrcON);
 }

 });
 $("#nav a").mouseout(function(){
 $(this).children("img").attr("src", imgsrc);
 });

	});

Codigo do Pre-Loader

var QueryLoader = {
	/*
 * QueryLoader Preload your site before displaying it!
 * Author: Gaya Kessler
 * Date: 23-09-09
 * URL: [http://www.gayadesign.com](http://www.gayadesign.com)
 * Version: 1.0
 *
 * A simple jQuery powered preloader to load every image on the page and in the CSS
 * before displaying the page to the user.
 */

	overlay: "",
	loadBar: "",
	preloader: "",
	items: new Array(),
	doneStatus: 0,
	doneNow: 0,
	selectorPreload: "body",
	ieLoadFixTime: 2000,
	ieTimeout: "",

	init: function() {
 if (navigator.userAgent.match(/MSIE (\d+(?:\.\d+)+(?:b\d*)?)/) == "MSIE 6.0,6.0") {
 //break if IE6
 return false;
 }
 if (QueryLoader.selectorPreload == "body") {
 QueryLoader.spawnLoader();
 QueryLoader.getImages(QueryLoader.selectorPreload);
 QueryLoader.createPreloading();
 } else {
 $(document).ready(function() {
 QueryLoader.spawnLoader();
 QueryLoader.getImages(QueryLoader.selectorPreload);
 QueryLoader.createPreloading();
 });
 }

 //help IE drown if it is trying to die :)
 QueryLoader.ieTimeout = setTimeout("QueryLoader.ieLoadFix()", QueryLoader.ieLoadFixTime);
	},

	ieLoadFix: function() {
 var ie = navigator.userAgent.match(/MSIE (\d+(?:\.\d+)+(?:b\d*)?)/);
 if (ie[0].match("MSIE")) {
 while ((100 / QueryLoader.doneStatus) * QueryLoader.doneNow < 100) {
 QueryLoader.imgCallback();
 }
 }
	},

	imgCallback: function() {
 QueryLoader.doneNow ++;
 QueryLoader.animateLoader();
	},

	getImages: function(selector) {
 var everything = $(selector).find("*:not(script)").each(function() {
 var url = "";

 if ($(this).css("background-image") != "none") {
 var url = $(this).css("background-image");
 } else if (typeof($(this).attr("src")) != "undefined" && $(this).attr("tagName").toLowerCase() == "img") {
 var url = $(this).attr("src");
 }

 url = url.replace("url(\"", "");
 url = url.replace("url(", "");
 url = url.replace("\")", "");
 url = url.replace(")", "");

 if (url.length > 0) {
 QueryLoader.items.push(url);
 }
 });
	},

	createPreloading: function() {
 QueryLoader.preloader = $("<div></div>").appendTo(QueryLoader.selectorPreload);
 $(QueryLoader.preloader).css({
 height: "0px",
 width: "0px",
 overflow:	"hidden"
 });

 var length = QueryLoader.items.length;
 QueryLoader.doneStatus = length;

 for (var i = 0; i < length; i++) {
 var imgLoad = $("<img></img>");
 $(imgLoad).attr("src", QueryLoader.items[i]);
 $(imgLoad).unbind("load");
 $(imgLoad).bind("load", function() {
 QueryLoader.imgCallback();
 });
 $(imgLoad).appendTo($(QueryLoader.preloader));
 }
	},

	spawnLoader: function() {
 if (QueryLoader.selectorPreload == "body") {
 var height = $(window).height();
 var width = $(window).width();
 var position = "fixed";
 } else {
 var height = $(QueryLoader.selectorPreload).outerHeight();
 var width = $(QueryLoader.selectorPreload).outerWidth();
 var position = "absolute";
 }
 var left = $(QueryLoader.selectorPreload).offset()['left'];
 var top = $(QueryLoader.selectorPreload).offset()['top'];

 QueryLoader.overlay = $("<div></div>").appendTo($(QueryLoader.selectorPreload));
 $(QueryLoader.overlay).addClass("QOverlay");
 $(QueryLoader.overlay).css({
 position: position,
 top: top,
 left: left,
 width: width + "px",
 height: height + "px"
 });

 QueryLoader.loadBar = $("<div></div>").appendTo($(QueryLoader.overlay));
 $(QueryLoader.loadBar).addClass("QLoader");

 $(QueryLoader.loadBar).css({
 position: "relative",
 top: "50%",
 width: "0%"
 });

 QueryLoader.loadAmt = $("<div>0%</div>").appendTo($(QueryLoader.overlay));
 $(QueryLoader.loadAmt).addClass("QAmt");

 $(QueryLoader.loadAmt).css({
 position: "relative",
 top: "50%",
 left: "50%"
 });
	},

	animateLoader: function() {
 var perc = (100 / QueryLoader.doneStatus) * QueryLoader.doneNow;
 if (perc > 99) {
 $(QueryLoader.loadAmt).html("100%");
 $(QueryLoader.loadBar).stop().animate({
 width: perc + "%"
 }, 500, "linear", function() {
 QueryLoader.doneLoad();
 });
 } else {
 $(QueryLoader.loadBar).stop().animate({
 width: perc + "%"
 }, 500, "linear", function() { });
 $(QueryLoader.loadAmt).html(Math.floor(perc)+"%");
 }
	},

	doneLoad: function() {
 //prevent IE from calling the fix
 clearTimeout(QueryLoader.ieTimeout);

 //determine the height of the preloader for the effect
 if (QueryLoader.selectorPreload == "body") {
 var height = $(window).height();
 } else {
 var height = $(QueryLoader.selectorPreload).outerHeight();
 }

 //The end animation, adjust to your likings
 $(QueryLoader.loadAmt).hide();
 $(QueryLoader.loadBar).animate({
 height: height + "px",
 top: 0
 }, 500, "linear", function() {
 $(QueryLoader.overlay).fadeOut(800);
 $(QueryLoader.preloader).remove();
 });
	}
}