SSRS report times out within reportviewer on web form

I have an SSRS report that's called within an web form via the ReportViewer control. It's a huge report, which now takes about 7 minutes to query (I can't try to work to reduce the time of this query because my company won't give me permissions to modify this stored procedure).

Due to this long query time, the SSRS report is timing out after 90 seconds on our website. If I go to the Report Manager & run the report, it's fine. It only times out on the website. I've tried adding "AsyncPostbackTimeout=600" to the scriptmanager for the ReportViewer control but no luck. Here's the code for that:

<asp:ScriptManager ID="ScriptManager1" runat="server" ScriptMode="Release" AsyncPostBackTimeout="600">
<div >

    <rsweb:ReportViewer ID="ReportViewer1" runat="server" Font-Names="Verdana" 
        Font-Size="8pt" ProcessingMode="Remote" Height="480px" Width="100%">
        <ServerReport ReportPath="the report path" 
            ReportServerUrl="http://ourserver/reportserver" />

I've also added the following line to my web.config but again, no luck:

<httpRuntime maxRequestLength="1000000" executionTimeout="600"/>

I went through the code & there's no place else we're specifying timeout time. I should also add that this is only happening on our Windows server (I believe it's 2008). When I try it locally, it works fine.

Any suggestions as to what else I can try?

Thanks in advance.

Without modifying the Sproc itself you can add indexes (if that will improve performance) or add a Plan Guide if the Sproc is using a rubbish query plan - i.e. you can force a Query Plan on the Sproc without actually having to change the SProc itself