We always not matter how the code is optimized there is always a complaint from the user that the reports are slow from their perspective, especially if they run the reports the first time. The performance is much better though when the report is re-run again, as we say it the SSRS has warmed up to service the report requests. So it is exactly what has been incorporated in the 2012 R2 CU7 and above, there is a new report "warm up" class called the SSRSReportServerWarmup.
The warm up class will connect and load the AX assemblies to reporting server cache which cuts up a lot of time when the users will actually run the report. However you would still need to run this class as a batch job after the Reporting server restart to get the performance gain.
The best thing about the class is it is extendable and you would be able to even run up specific reports to further the performance improvements in loading and caching the assemblies before the user actually runs the report. Please do read up on the extensibility and SSRSReportServerWarmup class at the TechNet and the AX support site
Do incorporate and demonstrate to the client on the performance gains made in the SSRS report loading if they have any complaints!