Hi,
Each tab contains a GridView will cause 6 GridView loaded in once time. That's too much data need to render and load, so the speed is slow.
You can load only one gridview in the default tab, other gridview can be loaded when the related tab panel is selected. Meanwhile, please put the updatepanel into tab panel to wrap the gridview. 6 tab panel have 6 updatepanel with 6 gridview. Every tab panel selecting will cause one of gridview update that sound more efficient.