Website owners can take many different actions to optimize and accelerate page load times, ultimately improving both the real and perceived user experience for visitors. The following is an excerpt from a Radware report - Spring 2015 State of the Union for Ecommerce Page Speed & Web Performance:

1. Consolidate JavaScript and CSS

Consolidating JavaScript code and CSS styles into common files that can be shared across multiple pages should be a common practice. This technique simplifies code maintenance and improves the efficiency of client-side caching. In JavaScript files, be sure that the same script isn’t downloaded multiple times for one page. Redundant script downloads are especially likely when large teams or multiple teams collaborate on page development.

2. Minify Code

Minification, which is usually applied to scripts and style sheets, eliminates non-essential characters such as spaces, newline characters, and comments. A correctly minified resource is used on the client without any special processing, and file-size reductions average about 20%. Script and style blocks within HTML pages can also be minified. There are many good libraries available to perform minification, often along with services to combine multiple files into one, which additionally reduces requests.

3. Enable Keep-Alives

Enabling keep-alives is one of the easiest "low hanging fruit" on the performance optimization tree, yet a significant number of sites fail to do this. TCP connection is the process by which both the user and the server send and receive acknowledgment that a connection has been made and data can begin to be transferred. Too many TCP connections will slow down your site. It’s not easy to speed up TCP connection, but you can control how many times the connection takes place. To enable keep-alives, make sure you have the proper configuration on your servers and load balancer.

4. Compress Text

Compression technologies such as gzip reduce payloads at the slight cost of adding processing steps to compress on the server and decompress in the browser. These operations are highly optimized, however, and tests show that the overall effect is a net improvement in performance. Text-based responses, including HTML, XML, JSON (JavaScript Object Notation), JavaScript, and CSS, can all be reduced in size by as much as 70%.

5. Sprite Images

Spriting is a CSS technique for consolidating images. Sprites are simply multiple images combined into a rectilinear grid in one large image. The page fetches the large image all at once as a single CSS background image then uses CSS background positioning to display the individual component images as needed on the page. This reduces multiple requests to only one, significantly improving performance.

6. Compress Images

Image compression is a performance technique that minimizes the size (in bytes) of a graphics file without degrading the quality of the image to an unacceptable level. Reducing an image’s file size has two benefits:

• Reducing the amount of time required for images to be sent over the internet or downloaded

• Increasing the number of images that can be stored in the browser cache, thereby improving page render time on repeat visits to the same page.

7. Reformat Images

Inappropriate image formatting is an extremely common performance culprit. An image that is saved to the wrong format can be several times larger than it would be if saved to the optimal format. Images with unnecessarily high resolution waste bandwidth, processing time, and cache space. As a general rule of thumb, these are the optimal formats for common image types:

• Photos – JPEG, PNG-24

• Low complexity (few colors) – GIF, PNG-8

• Low complexity with transparency – GIF, PNG-8

• High complexity with transparency – PNG-24

• Line art – SVG

8. Ensure That Feature Images Are Optimized to Load Early and Quickly

As discussed earlier in this report, site owners should be aware of the usability consequence of delaying the rendering of feature content: a user who experiences instantaneous page rendering spends 20% of their viewing time within the feature area of a page, whereas a user who endures an eight-second download delay spends only 1% of their total viewing time looking at the featured space on a page.

9. Rethink the Design and Location of Call-to-Action Links in Feature Graphics

While the accepted design convention has been to position CTA buttons at the bottom of feature banners, this convention does not always serve the best interests of users or site owners, as shoppers must wait for the image to fully render before taking their next action on the page. The simplest solution: Reposition the CTA.

10. Defer Rendering "Below the Fold" Content

Ensure that the user sees the page quicker by delaying the loading and rendering of any content that is below the initially visible area, sometimes called "below the fold." To eliminate the need to reflow content after the remainder of the page is loaded, replace images initially with placeholder tags that specify the correct height and width.

11. Defer Loading and Executing Non-Essential Scripts

Many script libraries aren’t needed until after a page has finished rendering. Downloading and parsing these scripts can safely be deferred until after the onload event. For example, scripts that support interactive user behavior, such as "drag and drop," can’t possibly be called before the user has even seen the page. The same logic applies to script execution. Defer as much as possible until after onload instead of needlessly holding up the initial rendering of the important visible content on the page.

The script to defer could be your own or, often more importantly, scripts from third parties. Poorly optimized scripts for advertisements, social media widgets, or analytics support can block a page from rendering, sometimes adding precious seconds to load times.

12. Use AJAX for Progressive Enhancement

AJAX (Asynchronous JavaScript and XML) is a technique for using the XHR (XMLHttpRequest) object to fetch data from a web server without refreshing the page where the code is running. AJAX enables a page to display updated data in a section of a page without reconstructing the entire page. This is often used to respond to user interaction, but it can also enable your application to load a bare-bones version of a page quickly, and then to fill in more detailed content while the user is already viewing the page.

13. Preload Page Resources in the Browser

Auto-preloading is a powerful performance technique in which all user paths through a website are observed and recorded. Based on this massive amount of aggregated data, the auto-preloading engine can predict where a user is likely to go based on the page they are currently on and the previous pages in their path. The engine loads the resources for those "next" pages in the user’s browser cache, enabling the page to render up to 70% faster. Note that this is a data-intensive, highly dynamic technique that can only be performed by an automated solution.

14. Implement an Automated Web Performance Optimization Solution

While many of the performance techniques outlined in this section can be performed manually by developers, handcoding pages for performance is specialized, time-consuming work. It is a never-ending task, particularly on highly dynamic sites that contain hundreds of objects per page, as both browser requirements and page requirements continue to develop. Automated front-end performance optimization solutions apply a range of performance techniques that deliver faster pages consistently and reliably across the entire site.