We experienced similar performance problems as described here. Loading portal pages took more than 30-40 secs, there were no CPU or SQL issues. Our on-prem Power BI Report Server (v. Jan 2019) is used by our co-workers in various locations/offices and behind a Amazon Route 53 DNS using a specific hostname. Then I got pointed in the right direction by using developer tools in Chrome returning some 401 http errors in the network tab.
It turned out all I had to do is add the hostname we are using to the registry of the Power BI RS and reboot the server. Please check the following kb article, for us Method 1 solved our problems.