SQL Server Reporting Services taking a long time before requesting report

Question

I am running a SQL Server 2016 instance with Reporting Services installed, and am seeing a strange "slowness" to my reports.

The report header and parameters load immediately, but then the report itself displays a white screen for ~15-20 seconds, before the loading wheel appears and the report renders.

Looking at ExecutionLog3, the report itself is only taking ~1140 milliseconds to get data, process and render. This seems correct, as that is about as long as the loading wheel appears on screen, but doesn't account for the ~15-20 seconds of white screen.

StartTime               | EndTime                 | TimeDataRetrieval | Time Processing | TimeRendering | ByteCount | RowCount
------------------------|-------------------------|-------------------|-----------------|---------------|-----------|---------
2019-03-14 10:29:53.057 | 2019-03-14 10:29:54.213 | 492               | 345             | 302           | 85370     | 4467 

**I did another test, The start time shown in ExecutionLog3 matches with when the loading circle appears, not when the parameters first appear 15 seconds earlier.

What is even stranger, adding &RC:Toolbar=False&rs:ClearSession=true to the report URL hides the parameter window, but that ~15 second wait before the report starts to load goes away, and the report takes ~1 second from clicking the link to report rendered. I have put this on some reports, but others need to have parameters visible to the user.

SQL Version:

Microsoft SQL Server 2016 (SP2) (KB4052908) - 13.0.5026.0 (X64) Microsoft Corporation  Standard Edition (64-bit) on Windows Server 2016 Standard 10.0 <X64> (Build 14393: ) (Hypervisor) 
Microsoft SQL Server Reporting Services Version 13.0.5026.0 

Answer 1

Perhaps the Internet is locked down?

SSRS 2016 uses a CDN-served JavaScript library for diagnostics.

Disabling usage tracking to Microsoft for the reporting services instances resolves the issue with "Application Insights" loading.

It can be done using the SQL Server Error and Usage Reporting tool:

More info: https://twitter.com/RiccardoMuti/status/799350487426289668

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And, another suggestion is to delete and republish the RDL file. It can work, if there was in place upgrade of SSRS and the catalog table contains RDL in an outdated version.