the RadPivotGrid is a great control for viewing large amounts of data, but with 25,000 records any control would slow down the browser and / or break. Are you trying to display all 25,000 records at once? It might help if you enabled paging so only a specified amount will be displayed at once. Also, are you editing these records in the PivotGrid? If not, it might benefit you to set EnableViewState to 'False'.
There is a Webinar on tv.telerik.com
that is about optimizing a RadGrid control. The video is over an hour long, but it has some great tips on optimizing the speed that your Grid is loaded into your browser. The demos start at about 20 minutes in. Hopefully this information helps you optimize your RadPivotGrid.