1. Try to split initial payload .. first load all required js to load page then other scripts/things.
Lazyload,ajaxload,load scripts in parrller,load script after on windowload, these are different things and you must know about it.When the browser starts downloading an external script, it won’t start any additional downloads until the script has been completely downloaded, parsed, and executed. So load external scripts asynchronously only. we also need to care about inline JS, here we can use callback functions,timers,on window load etc see following chart , here few techniques are listed to load JS scripts.
2. Flushing the Document Early:-
As the server parses the PHP page, all output is written to STDOUT. Rather than being sent immediately, one character, word, or line at a time, the output is queued up and sent to the browser in larger chunks. This is more efficient because it results in fewer packets being sent from the server to the browser. Each packet sent incurs some network latency, so it’s usually better to send a small number of large packets, rather than a large number of small packets. Calling flush() causes anything queued up in STDOUT to be sent immediately. How- ever, simply flushing STDOUT isn’t enough to achieve the type of speedup experienced in the preceding example. The call to flush has to be made in the right place. idea is , flush before a long time taking process, it can be divide as module wise also like header,footer,sidebar,widgets etc.
3. Repaint and Reflow is costly:-
A repaint occurs when a visual change doesn't require recalculation of layout Changes to visibility, colors (text/background), background images, etc.
A reflow occurs when a visual change requires a change in layout Initial page load
browser resize,DOM structure change,layout style change layout information retrieved
4. Eliminate unnecessary cookies:-
Keep cookie sizes as low as possible to minimize the impact on the user response time,Be mindful of setting cookies at the appropriate domain level so other sub-domains are not affected
5. DOM manipulations are the slowest,Never update the DOM, if that's not possible, at least do it as infrequently as possible. Bunch up your updates to the DOM and save them for a later time.
6. Clone the DOM node you want to work with. Now you will be working with a clone of the real node, and the cloned node doesn't exist in the DOM. Updating the cloned node doesn't affect the DOM. When you are done with your manipulations, replace the original node with the cloned node.
7. However, note that the performance problems in point 4, because of the content and rendering reflow that the browser has to do. You might get similar benefits by simply hiding the element first, making the changes, and then showing it.