Time to First Byte (TTFB) とは何ですか?
The time to first byte (TTFB) measures the duration between when the browser requests a resource (like a webpage) and when the server sends the first byte of information of that resource. It is helpful for evaluating the load speed of a webpage but is not a コアウェブバイタル metric.
The time to first byte is the sum of the time it takes a series of requests to go from the browser to the server and back. The times that are added together to calculate the time to first byte include the following:
- リダイレクト 時間
- Service worker startup time
- DNS lookup time
- Connection and TLS negotiation time
- The time it takes the data to return to the browser
Many bloggers use the time to first byte to evaluate the speed of a server. However, it could be affected by non-server-related issues such as redirect time and the time it takes for the data to return to the browser. So, the time to first byte is not a definite indicator of a server’s speed.
What is a Good Time to First Byte Score?
A good time to first byte score is 800 milliseconds or below. Tools that report on the time to first byte metric typically classifies the score into three categories:
- Good – 800 milliseconds or less
- Needs improvement – Above 800 milliseconds and 1800 milliseconds
- Poor — Over 800 milliseconds
Importance of the Time to First Byte Metric
The time to first byte is helpful for evaluating the load speed of a webpage. Specifically, it helps to identify issues that occur in the early stages of the webpage loading.
This makes it helpful for diagnosing problems with other load speed metrics like the largest contentful paint (LCP) と first contentful paint (FCP), which both include the time to first byte in their calculations.
So, the time to first byte metric impacts both metrics. As a result, if the time to first byte is poor, then both metrics are likely to be poor. Similarly, improving the time to first byte will result in an improvement in both metrics.
The time to first byte and first contentful paint are load speed diagnostic metrics, while the largest contentful paint is a Core Web Vitals metric. Google uses the Core Web Vitals and several other signals and metrics like HTTPS usage と mobile-friendliness to determine the page experience of a webpage. Page experience is a ranking factor.
How to Measure the Time to First Byte
You can measure the time to first byte using the Web Vitals extension. The extension measures the time to first byte of any page you visit, so it allows you to inspect the time to first byte of a webpage in real-time.
Using a Chromium-based browser like Chrome or Brave, head to the Web Vitals拡張機能 on the Chrome web store and click Chromeに追加. (The wording may differ depending on the browser.)
Once done, head to a webpage and click the Web Vitals icon. The color and shape of the icon may differ depending on the Core Web Vitals score of the webpage.
The time to first byte of the webpage will be displayed.
