First asp.net core app. fast localy, 30s loadtime on internet

Hi. I started with asp.net applications and iis like a month or two ago so bear with me here. This is a initial problem after finally getting internetaccess to the page. The problem is that everything works fine localy, the application is very fast and responsive. But when I access it  through the dns beta.dayzintenz.com its superslow and sometimes it even times out. Its a updated ASP.NET core application. Im using a TP-link router, and settings should be mostly default.

What could be wrong here? I have 200 down and 40 up in the tests I do on my server, and the computer has almost no workload when dealing with the requests. Is there any debugging tools? I am using windows server 2012 r2. Basically any kind of help with debugging these kind of problems would be appreciated. Even basic cmd commands.

Thank you in advance.