Slow rendering, caching issues, Cannot use Session-based caching mechanism while session is disabled, Session-based caching, session disabled

If you got "Cannot use Session-based caching mechanism while session is disabled.", it means that HttpContext.Current.Session is null.
In order to avoid this issue you need to 
A) Specify the following attribute for your service:
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
B) Web.Config should have the following setting:
<serviceHostingEnvironment aspNetCompatibilityEnabled="true" multipleSiteBindingsEnabled="true" />
If you open package sniffer and find out that there are many requests with the following suffix "...FullRequest" or if you open log file and find a lot of "Document is out of cache" messages, then you should follow the instructions below:
1. Switch on IIS sessionState;
2. Switch on browser cookies.

Please check requests which are passed to ReportService (WCF). 
To check requests, you need to do the following:
- To see requests in IE -
- To see requests in FireFox -
- To see requests in Google Chrome -
- Also you can use fiddler -

If your site settings can't be changed to support session state, then you may implement your own caching algorithm:

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