Facebook like many other Western social media platforms in banned in China. As a result, files from Facebook servers can’t be loaded in Mainland China.
The advice for Facebook Pixel is therefore very simple. If possible, don’t use it.
If you have to use it, ideally load it at the very end of the website (in the footer area). This way, the content of the website gets loaded and shown first, then, at the end, Facebook Pixel gets loaded.
This might still make the website load slowly in Mainland China, but the main content of the site should become visible within a reasonable time.