YSlow analyzes web pages and why they’re slow based on Yahoo!’s rules for high performance web sites.
As YSlow is Open Source, there’s every chance it could outlive Yahoo! itself!
A speed analysis metric
Using speed analysis tools like GTmetrix, or your own browser based tools, one can generally see YSlow and Google PageSpeed are the two defacto standards when it comes to objectively measuring pageload times.
It is not all about scores!
It should be noted, however, that this score is a guide, and actual speed is a more important consideration. Sometimes the fastest setup delivers a slightly lower “score” – and this should be borne in mind when performing page speed tests.
Also, you might score 100 on Yslow, and have a site which takes 5 seconds to load. In such cases, a “worse” score on a site which takes 1 second to load would be better. Best would be a fast site which has a good score – the score means it is (close to) optimal, and you aren’t wasting resources, for example, loading images which are larger than they need to be, or failing to browser cache resources which do not change.