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HTTP/2 302 server: nginx date: Thu, 15 Jan 2026 21:28:50 GMT content-type: text/plain; charset=utf-8 content-length: 0 x-archive-redirect-reason: found capture at 20091223024302 location: https://web.archive.org/web/20091223024302/https://developer.yahoo.com/yslow/ server-timing: captures_list;dur=0.694778, exclusion.robots;dur=0.044186, exclusion.robots.policy;dur=0.034857, esindex;dur=0.011095, cdx.remote;dur=186.194287, LoadShardBlock;dur=191.111320, PetaboxLoader3.datanode;dur=140.706504, PetaboxLoader3.resolve;dur=12.766606 x-app-server: wwwb-app210-dc8 x-ts: 302 x-tr: 445 server-timing: TR;dur=0,Tw;dur=0,Tc;dur=0 set-cookie: wb-p-SERVER=wwwb-app210; path=/ x-location: All x-as: 14061 x-rl: 0 x-na: 0 x-page-cache: MISS server-timing: MISS x-nid: DigitalOcean referrer-policy: no-referrer-when-downgrade permissions-policy: interest-cohort=() HTTP/2 200 server: nginx date: Thu, 15 Jan 2026 21:28:50 GMT content-type: text/html; charset=UTF-8 x-archive-orig-date: Wed, 23 Dec 2009 02:43:02 GMT x-archive-orig-p3p: policyref="https://info.yahoo.com/w3c/p3p.xml", CP="CAO DSP COR CUR ADM DEV TAI PSA PSD IVAi IVDi CONi TELo OTPi OUR DELi SAMi OTRi UNRi PUBi IND PHY ONL UNI PUR FIN COM NAV INT DEM CNT STA POL HEA PRE LOC GOV" x-archive-orig-cache-control: private x-archive-orig-connection: close x-archive-guessed-content-type: text/html x-archive-guessed-charset: utf-8 memento-datetime: Wed, 23 Dec 2009 02:43:02 GMT link: ; rel="original", ; rel="timemap"; type="application/link-format", ; rel="timegate" content-security-policy: default-src 'self' 'unsafe-eval' 'unsafe-inline' data: blob: archive.org web.archive.org web-static.archive.org wayback-api.archive.org athena.archive.org analytics.archive.org pragma.archivelab.org wwwb-events.archive.org x-archive-src: portuguese-web-archive-AWP62009-24/IAH-20091223021226-05277-p2.arquivo.pt.arc.gz server-timing: captures_list;dur=0.656446, exclusion.robots;dur=0.017805, exclusion.robots.policy;dur=0.007491, esindex;dur=0.014739, cdx.remote;dur=10.866828, LoadShardBlock;dur=236.576939, PetaboxLoader3.datanode;dur=196.303311, PetaboxLoader3.resolve;dur=132.994308, load_resource;dur=171.698868 x-app-server: wwwb-app210-dc8 x-ts: 200 x-tr: 507 server-timing: TR;dur=0,Tw;dur=0,Tc;dur=0 x-location: All x-as: 14061 x-rl: 0 x-na: 0 x-page-cache: MISS server-timing: MISS x-nid: DigitalOcean referrer-policy: no-referrer-when-downgrade permissions-policy: interest-cohort=() content-encoding: gzip Yahoo! YSlow for Firebug

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YDN YSlow

Yahoo! YSlow

YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages. YSlow is a Firefox add-on integrated with the Firebug web development tool. YSlow grades web page based on one of three predefined ruleset or a user-defined ruleset. It offers suggestions for improving the page's performance, summarizes the page's components, displays statistics about the page, and provides tools for performance analysis, including Smush.it™ and JSLint.

How Do I Get Started?

Learn More

Best Practices

Click here to learn more about the best practices for making high performance web sites.

Smush.It™

Read more about how the Smush.it™ tool can help optimize web page image files.

Frequently Asked Questions

If you have questions about YSlow, check the frequently asked questions for the product.

Developer Community and Support

YSlow development is discussed in the Exceptional Performance group; this group was formed by Yahoo!'s Exceptional Performance team as a forum for communicating about web performance. Contact this group for assistance with YSlow questions, issues, or technical support.