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I make heavy use of the excellent jTemplates plugin for a small web app.

Currently I load all of the templates into the DOM on the initial page load.

Overtime as the app has grown I have gotten more and more templates - currently about 100kb worth.

Because my app is all ajax-based, there is never a need to refresh the page after the initial page load. There is a couple second delay at the beginning while all of the templates load into the DOM, but after that the app behaves very responsively.

I am wondering: In this situation, is there any significant advantage to using jTemplates processTemplateURL method to lazy load templates as needed, as opposed to just bulk loading all of the templates on the initial page load?

(I don't mind the extra 2 or 3 seconds the initial page load takes - so I guess I am wondering -- besides the initial page load delay, is there any reason not to load a large amount of html template data into the DOM? Does having a larger amount of data in the DOM affect performance in any way?)

Thanks (in advance) for your help.

Hussein
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Trev
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  • Not sure who your target user is, but a few seconds delay on a typical developer's machine could translate to something much longer on a "mainstream" box. – harpo Mar 23 '11 at 05:09
  • JavaScript performance is slow in old versions of IE so that could also be a contributing factor to the delay. What browser are you using? Also, how many of these templates are used at any one time? – Razor Mar 23 '11 at 05:26

3 Answers3

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According to yahoo Best Practices for Speeding Up Your Web Site article, They reommand not having more then 500-700 elements in DOM.

The number of DOM elements is easy to test, just type in Firebug's console:

document.getElementsByTagName('*').length

Read more http://developer.yahoo.com/performance/rules.html

Like a jar that contains 100 marbles 10 of which are red color. It is easy to spot and pick the 10 red marbles from a jar of 100, but if that jar contained 1000 marbles, it will take more time to find the red marbles. Comparing this to DOM elements, the more you have the slow your selections will be, and that will effect performance.

Hussein
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  • 700 dom elements seems too low an upper limit.To put this into context I ran the code you posted for facebook and it shows 2976 elements at once. – Akshat Jiwan Sharma Nov 11 '12 at 19:17
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Having whole bunch of extra DOM elements will not only affect your initial load time, it will also affect more or less everything on the page. JavaScript DOM queries will run slower, inserts will run slower, CSS will apply slower and so on, since the entire tag soup is parsed into DOM Tree by the browser, any traversal of that tree is going to be affected. To measure how much it's going to slow your page down, you can use tools like DynaTrace AJAX and run it once with your current code and once with your code with no templates. If you notice large difference between those two runs in terms of JavaScipt execution or rendering, you should probably lazy load (though 100kb is not that much, so you might not see significant difference).

Ilya Volodin
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You really should optimize your DOM in order to save memory and enhance speed. However, the key is to avoid premature optimizations.

What is your target platform? What browser(s) are your users most likely to be using?

For example, if you are targeting primarily desktop PC's and your users are running modern browsers, then you probably should prefer clarity and simplicity of your code.

If you are targeting desktop PC's but must support IE6, say, then having too many DOM elements will impact your performance and you should think in lines of optimizations.

However, if you are targeting modern browsers, but in areas with poor bandwidth (e.g. on a cruise ship etc.) then your bandwidth considerations may out-weight your DOM considerations.

If you are targeting iPhones, iPads etc., then memory is a scarce resource (as is with CPU), so you definitely should optimize the DOM. In addition, on mobile devices, you're going to give more weight in optimizing the AJAX payload due to bandwidth issues than with anything else. You'll give yet more weight in reducing the number of AJAX calls vs. saving on DOM elements. For example, you may want to just load more DOM elements in order to reduce the number of AJAX calls due to bandwidth considerations -- again, only you can decide on the right balance.

So the answer really is: depends. In a fast-connection, modern browser environment, no real need to prematurely optimize unless you DOM gets really huge. In a slow-connection, or mobile enviornment, overweight on bandwidth optimizations before DOM optimizations, but do optimize on DOM node count as well.

Stephen Chung
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