Given that you’re already using caching via W3TC, I’m thinking it might be your hosting. How is your site hosted?
I’m using a server on Aruba
Going to http://example.com, I see a 3.1 second TTFB, followed by a redirect to https://example.com which has a 6 second TTFB. Contact your host and ask them what’s going on. It might be an overloaded or underpowered server, a slow MySQL server, or some network issue.
According to the comments on your page, there’s some issue with page caching, too, so I recommend asking at https://ww.wp.xz.cn/support/plugin/w3-total-cache/#new-post so the plugin’s / theme’s developers and support community can help you with this.
Performance optimized by W3 Total Cache. Learn more: https://www.boldgrid.com/w3-total-cache/
Page Caching using disk: enhanced{w3tc_pagecache_reject_reason}
Lazy Loading
Database Caching 30/266 queries in 0.433 seconds using disk
Served from: www.xxxx.eu @ 2021-04-15 17:28:52 by W3 Total Cache
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