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Definition of Yslow

Yslow is an open source tool. It is used to analyze web pages and get suggestions for optimizing performance. When opening a web page to be tested, yslow analyzes it according to a set of predefined rules. As a result, the user receives information as well as a list of what is already ok in terms of web page performance and what could be improved.

Interpret results and improve scores

When the desired web page is searched, it is compared against a list of 23 rules. These rules are based on Yahoo’s “high performing websites”. The open source tool next evaluates the web page against these 23 rules and returns an overall score based on this average.

Each of the 23 rules has a different weighting. That is, some rules have a greater impact on the overall score than others. For example, limiting the number of HTTP requests made by the web page has a greater impact on the score than decreasing the size of the Cookies.

When the web page is run through GTmetrix, all the results and information can be viewed. First, you see an overall score. This is again the weighted average of the performance of the web page compared to all 23 rules.

The total score works just like the tests in school: 100% is a perfect score and 0% is the worst possible score.

Displaying the individual rules

The yslow overall score gives a good idea of how a web page is doing. The individual rules indicate where the core of the analysis is.

Data points:

This is the score a web page received for that particular rule.
This is the type of optimization, for example, whether the rule applies at the server level or to the CSS code of the web page.
This is the approximate weighting of each rule. A rule with a high priority has a greater impact than one with a low priority. The rules.js file on the SLow Github page shows how the score is calculated for each rule.


Yslow is not a comprehensive performance analysis. Rather, it focuses on the front-end performance of a website. However, it does not touch on any of the back-end elements of performance, such as quality of web hosting, database optimization, etc. These back-end elements are just as important to website performance, if not much more so.
Ultimately, all that is offered is a set of suggestions to improve the performance of the front-end elements. These suggestions are valuable and can help with optimization.

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