@AndreaPellegrin, it's difficult to help if you don't tell details.

Long loading times could be a DNS issue, missing CSS could be an issue with self-signed certificates or with wrong/missing CORS (Cross-Origin Resource Sharing) headers if a separate domain for static content is used. There are dozens other possible causes for such issues, too.

AndreaPellegrin wrote

If can help my external portal is in Apache Serve on Raspberry Pi 4, so i refer so:

http://192.168.1.118/login.html

This is obvious, but there could be a DNS lookup somewhere in the chain of requests and redirects, which probably causes delays in loading the portal page. That's what I suspected as the cause for delays, especially if the delays don't occur for any request, but only intermittently (b/c NX domain responses being cached). Just a guess.

As for static files, you should address them as usual in HTML, that's »/static-files/filename.css«, where static-files is your folder and filename.css is the name of your CSS file.