Yslow is a Web page component performance tester, from Yahoo, that provides some indication of what you should look at but provides too much information in a messy format and does not provide some of the critical detail. Only use Yslow with other tools.
Yslow runs in Firebug in Firefox and can run in other Web browsers. I tested a Drupal 7 web site in Firefox and Firebug. Some of the comparisons are to the information provided by Firebug.
Yslow produces an alphabetical grade with A the highest rating. The rating is based on rules you can replace with your own rules. Some of the rules supplied with Yslow appear to twist the ratings towards specific types of Web sites I rarely use or build while other combinations of rules appear to be aimed at selling Yahoo services. The priority assigned to the rules is not an order of priority I would use. The report often gives priority to things you cannot change because the decisions are made for reasons more important than performance.
F to B
The next problem with Yslow is the grade of B for a site that, for performance, I would grade as C or D. The site was a B when running Drupal 6, a slim theme, and fewer modules. Now it is at least a C-. I suspect Yslow is too easy.
There are reasons for using a CDN, content delivery network, and reasons to not use one. yslow always recommends using a CDN but misses major things we should look at before looking at a CDN. Yslow ignores massive duplicate files and other errors then says we should speed up the delivery of wasteful files by using a slightly faster server system. you should be looking at the reasons for downloading the files before looking at a CDN. The Web as a whole should look at why Twitter, Google, and LinkedIn create massive amounts of traffic around tiny logos that should do nothing until selected.
Avoid empty src or href
Yslow warns me to
Add Expires headers
Adding long cache expiries to some files makes sense because they are kept in the Web browser and are reused from page to page. Yslow complains about files with short expiries, no expiry, and immediate expiries. The immediate expiries are there for a reason, to deliver fresh content. The short expiries are in fact long expiries in Web time. The files without expiries are like that to let Web browsers default to saving files across a session and no longer.
Compress components with gzip
You switch compression on in your Web server. I switch it on in Apache. Yslow is warning me about it as if there is a problem. I am starting to detect a pattern. If Yslow warns you about a problem but does not list an example, it may mean the problem does not exist for the page test. yslow should really be rewritten from the ground up by someone who knows something about designing applications for use by someone other than the developer.
I will skip all the reports where there are no examples listed.
Reduce DNS lookups
The message warns you to reduce the number of domain names used to four or less then lists four domain names. This is one warning that could be smarter.
Cache external files
Use cookie-free domains
This part of the report introduces an interesting concept that might save a very small amount of time. The main effect would be on retrieving millions of tiny icons. Using multiple icons in one image is easier and faster. Again, if Yslow actually looked at your page and worked out the changes with the most benefit, this part of the report would be empty or recommend other techniques.