In the May (then with the July mini update) and with the September update, I'm seeing slow loading of SSRS Reports. Mainly, when Managing, the Subscriptions and the Parameters, but also just loading the page. I seems to be the Website in a sleep mode and its waking up. For most Website, with IIS, you can set the Interval to be always on. With Power Bi Report Server, I don't know if thats set in some config file, registry, or not possible. All of the subsciptions go out on time, but its after an hour or so, it take a while to get a report to load. Once one report loads, all the reports load quickly so its not not one report in particular. I have not tried a PBI report to seee if it load right off or not.
I can confirm your findings. I have been testing the May 2021 release in my TEST environment but hadn't tried htting the My Subscriptions page first thing. I have seen the same behavior with SSRS on SQL Server 2008 R2. The cache on the server is dumped after a period of time to conserve resources and the first person in each day causes things to be loaded into memory that speeds up the subsequent hits on the site. I will check out the report server config and logs.
2021-10-04 07:28:51.9843|INFO|87|ui|AAD returned 'System.Net.HttpWebResponse'. You may need to reregister your application with PowerBI in the RS Config Tool.| RequestID = s_148d59f6-73b7-4ace-a87a-2c1474dc2738 2021-10-04 07:28:51.9843|INFO|87|ui|WebException: System.Net.WebException: The remote server returned an error: (401) Unauthorized. at System.Net.WebClient.UploadValues(Uri address, String method, NameValueCollection data)
Thats seems strange since I'm not sure how the Power BI Report Server running locally has anything to do with the Cloud version. In any case, I went to the RS Config Tool (Report Server Configuration Manager) , selected Power BI Service (Cloud) and selected UpdateRegistration. I logged in with my Domain account and when I was finished, it wanted to restart the service. It took a while to restart the service and there seemed to have been an improvement, but I'll need to check tommorrow after setting over night. Its strange that it gets hung up waiting to authenticate to the Cloud, then it moves on since the page will eventually load. I could see if someone was using the Hybrid Cloud features, but not for just local reports. Seems like a bug to me.
I can also confirm that I do not have my on-prem Report Server instance registered with the Power BI service. From what I read about it, it only allows on-prem report items (i.e. visuals) to be pinned to a hosted service dashboard. This means the performance slowness on initial startup for me was not caused by any kind of link or registration to the cloud. I'm betting on the memory dumping/caching being the cause.