Understanding Page Speed performance scores

For each rule, Page Speed gives specific suggestions for improvement, and gives the page a "score" according to a heuristic that weighs a number of factors. It also gives the page a total performance score.

Understanding per-rule scores

For each rule, there are two kinds of scores: a numeric score, which is a "grade" out of 100; and a color-coded score (green, yellow, or red). The numeric score is a "raw" score that indicates how the page performed on that rule, using some quantitative measure, such as, for example, the total number of DOM elements, or the number of downloaded files. The color-code score factors in the numeric score and the rule's weight, which is a composite of the potential impact of the rule (based on our experience), and its difficulty of implementation.
This means that there is not a one-to-one mapping between a numeric score and a color code. For example, a score of 0/100 could be translated into a yellow color code if the weight of the rule is not very high. Therefore, you should always refer to the color-code score as the authoritative one.
Here's how to interpret the color-code scores:

• High priority. These suggestions represent the largest potential performance wins for relatively little development effort. You should address these items first.
  
• Medium priority. These suggestions may represent smaller wins or much more work to implement. You should address these items next.
  
• Working fine or low priority. If suggestions are displayed, as indicated with a + sign, they probably represent minor wins. You should only be concerned with these items after you've handled the higher-priority ones.
• Informational messages only. Either these items don't apply to this page or there was a problem in running the test.
  Tip: If your results show a large number of informational messages, this is likely because you tried to analyze the page before it was fully loaded. Click Refresh Analysis to rerun the analysis.

Understanding the total score

Page Speed also gives you a total numeric and color-code score. The numeric score is calculated as the total numeric score for the page, divided by the total weight of all rules (excluding rules for which you got a "blue"/informational-only result, which are non-scoring). The color-code score is calculated using a heuristic based on the number of green, yellow, and red results.