When you run a mobile-first index, Google will not consider the speed of the mobile page load as a ranking factor. This was stated by spokesman John Muller search.
Google now accounts for only PageSpeed desktop pages in the rankings, including mobile.
During a live video John Mueller asked a few questions on this subject:
Q: Is the mobile download speed affect rankings on desktop?
-At the moment, we do not take into account the rate for mobile sites. So far, trying to understand how it is better to implement it in practice. But now we do not specifically take into account this factor.
Question: You did not take into account the speed of the mobile ranking, but take into account in the desktop?
-No, we do not take into account the speed of mobile sites.
Listen to John Mueller questions and their answers can be in the video stream at the level of 20:50:
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