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1. Minify and compress Javascript & CSS

2. Optimize images

3. Reduce HTTP requests

4. Caching

General tips

Web Performance (Pt. 1)

Reduce HTTP requests

The fastest and best optimized resource is a resource not sent.

Compression

1. Concatenate JS/CSS files

2. Minify JS/CSS

3. Enable Gzip Compression

4. Use less JS/CSS

HTTP caching

Web Performance Pt. N?

Optimize images

Web Performance Pt. N?

Chrome DevTools

Critical rendering path

https://developers.google.com/web/fundamentals/performance/critical-rendering-path/

• Minimize the number of critical resources
• Minimize the number of critical bytes
• Minimize the critical path length

Optimize the order of  loading

Chrome DevTools

async

Async resources unblock the document parser and allow the browser to avoid blocking. 

CSS in the head

All CSS resources should be specified as early as possible within the HTML document such that the browser can discover the <link> tags and dispatch the request for the CSS as soon as possible.

Style recalculations

Expensive styles

Unefficient selectors

1. Reduce the complexity of your selectors

2. Use a class-centric methodology like BEM

Use local variables for caching

var $window = $( window ),
    $document = $( document ),
    $footer = $( '#footer' ),
    $sidebar = $( '#sidebar' ),
    $images = $( 'img' );

Layout

What causes relayouts?

http://goo.gl/lPVJY6

Layout Thrashing occurs when JavaScript violently writes, then reads, from the DOM, multiple times causing document reflows.

// Read
var h1 = element1.clientHeight;

// Write (invalidates layout)
element1.style.height = (h1 * 2) + 'px';

// Read (triggers layout)
var h2 = element2.clientHeight;

// Write (invalidates layout)
element2.style.height = (h2 * 2) + 'px';

// Read (triggers layout)
var h3 = element3.clientHeight;

// Write (invalidates layout)
element3.style.height = (h3 * 2) + 'px';

https://csstriggers.com/

What causes relayouts?

Identifying layout problems

Chrome DevTools Timeline

Chrome DevTools Paint Flashing

Identifying layout problems

1.

2.

Operate with off-document nodes

• Hide the element, apply changes, and show it again.
• Use a document fragment to build a subtree outside of the live DOM and then copy it to the document.
• Copy the original element into an off-document node, modify the copy, and then replace the original element once you’re done.

requestAnimationFrame

The Window.requestAnimationFrame() method schedules a function to be executed at the next frame

fastDOM

https://github.com/wilsonpage/fastdom

Use debouncing

function debounce(func, wait, immediate) {
	var timeout;
	return function() {
		var context = this, args = arguments;
		var later = function() {
			timeout = null;
			if (!immediate) func.apply(context, args);
		};
		var callNow = immediate && !timeout;
		clearTimeout(timeout);
		timeout = setTimeout(later, wait);
		if (callNow) func.apply(context, args);
	};
};

var myEfficientFn = debounce(function() {
	// All the taxing stuff you do
}, 250);

window.addEventListener('resize', myEfficientFn);

Use CSS classes

Instead of changing styles, manipulate pre-defined CSS classes

Web Workers

Multi-threading :)

Cannot work with DOM :(

Painting

http://admrzn.ru/

translateZ

.our-element {
    transform: translateZ(0);
}

will-change

.some-element {
    will-change: transform;
}

pointer-events: none

.avoid-events {
    pointer-events: none;
}

Third-party libraries

Web Performance Pt. N?

Time to load

https://aerotwist.com/blog/the-cost-of-frameworks/