I noticed that you have also submitted a support ticket with the same query. I suggest we continue the conversation there. Nevertheless, I will paste the answer here, it can be helpful to someone observing similar behavior.
The issue that you are describing could be observed if you are loading too many records in RadGrid without enabled paging. Having huge amount of records with that amount of columns will generate too many DOM elements and it will be difficult for the browser to handle the scrolling (especially if you have frozen columns and enabled static headers). Note that the performance will be worst in IE browsers, most noticeable in Compatibility View and IE 7.
If your scenario matches the above, the only option for increasing the performance is to enable the paging functionality of RadGrid or if applicable, enable Virtual Scrolling. However, keep in mind that with 50 columns you need to limit the amount of records per page, so that less DOM elements are created for the RadGrid.
With that said please avoid submitting duplicate threads. This will enable us to keep better track of your support history and provide answers faster.
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.