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 an important consideration. Sometimes the fastest setup delivers a slightly lower “score” – and this should be borne in mind when performing page speed tests.