XF 2.2 jquery-3.5.1

Updating jQuery won't fix this, it's rather that you have another piece of code which is merely asking jQuery to make a synchronous request. We don't do this in the core (barring one place where it's necessary on ancient browsers, but that doesn't trip page speed as demonstrated by running it against this very forum).
If you are using my Cloudflare addon and have guest page caching enabled (where pages are cached in Cloudflare’s data centers), that might cause what you are seeing.

Specifically, since the CSRF token isn’t stored for cached pages, it will make a quick AJAX request to get a CSRF token for the guest user. That single request is intentionally run in a way that blocks additional AJAX requests until it’s done. It’s done that way because additional AJAX requests can’t run until the CSRF token is returned from that first request.

The speed gain of serving HTML pages from the closest Cloudflare data center far outweighs the downside of doing the quick fetch for a CSRF token.

…or maybe you aren’t using Cloudflare with guest page caching. 🤷🏻‍♂️
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