Runtime – avatars are being cached on the first-page load, this method slows the first-page loading, but images are cached immediately.
Cron job – images are being cached every 3 hours (default).
So in your case, you should use the Runtime method.
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@asti Thank you for your answer I just tried it and I see that gtmetrix shows the same problem. Maybe I'm wrong, but this problem was not there before in version 7.0.0 and I used the Cron job method