Hi Computer Surgeon,
As a first step, make sure that you set the RenderSelectedPageOnly
property of the RadMultipage control to true, set AutoPostBack for RadTabStrip and ajaxify them both using RadAjaxManager as shown in this KB article:
Thus merely the controls which are in the active page view will be rendered/bound to data and the operations will be perform with asynchronous requests.
Furthermore, consider putting into practice the hierarchy load modes
for the hierarchical grids to speed up the loading time as much as possible.
Additionally, review the performance optimization tips available in the online resources linked below:
(Chapter "Performance tips and tricks")
the Telerik team