power bi report server very slow

Anonymous 

Also below is the security policy associated with it:-

CREATE SECURITY POLICY [dbo].[SecurityPolicy]
ADD FILTER PREDICATE [dbo].[func_PLAN_KEY_FILTER]([PLAN_KEY]) ON [dbo].[ABC]
WITH (STATE = ON, SCHEMABINDING = ON)
GO

So you're running two different queries (or query plans at least), one for regular users, one for the system account. 

I would attach a profiler trace to the SQL backend and then run my front end as the service account and again as an end user (test user) and see what the differenece is in response time. If its fast as the system user then that suggests its not a rendering problem/issue but a data query issue. So assuming its slow when a user identity is passed in you could look at optimising the SQL

Looking at your function there are a couple of options.

You could create an index on your SECURITY_GROUP table that is keyed on the LOGIN_NAME and includes the GROUP_NAME, ideally you could cluster the table on those two columns but that may have issues if you have dupliacte rows. You may also need an index on your PROGRAMMES table on the GROUP_NAME including the PLAN_KEY. This would leave your function unchanged.

The other is to expand the whole thing. So turn your JOIN into an indexed VIEW (SCHEM_BINDING will be required) and then index the view on LOGIN_NAME and PLAN_KEY. You can then query the view using an EXISTS test which will be quicker. Something like

RETURN SELECT 1 AS AccessRight
WHERE EXISTS(SELECT 1
             FROM Indexed_view 
             WHERE ( PLAN_KEY = @PLAN_KEY 
                AND  LOGIN_NAME = system_user )
                OR ( system_user = 'ServerName\ServiceAcctname'  )