Cache preload seems not working

Hello, I’m using W3TC on https://labibledelentrepreneur.com and I think the preloading feature doesn’t work.

Here are my cache preload settings :
Update interval : 607
Pages per interval : 15
Sitemap URL : https://labibledelentrepreneur.com/sitemap_index.xml (with WordPress SEO as recommended

I’ve tried 60 (interval) with 500 pages, but that was the same result :
Somes pages are slow, but once you’ve visited it one time, that’s very fast. And a couple of hours later, the same page is slow as if the cache was deleted.

I noticed that sometimes, categories are cached, but subcategories & articles aren’t.

Thank you for your help : )
Pierre

https://www.ads-software.com/plugins/w3-total-cache/