How many records do you have in the treelist? If it is a large number, a solution to this problem will be available in the next release of RadControls with a new load mode for the treelist which improves performance with large datasets.
If the records are not that many, there could be some problem with your databinding logic, so sharing the code that you use for binding the treelist could help.
As for the second problem, the TreeListTemplateColumn does not support such functionality out of the box. There could be anything in it, so the behavior of the controls inside is controlled only by the logic which you provide.
If you want to have such functionality that checks all child rows of the currently selected one, you could use a TreeListSelectColumn with recursive selection enabled (in the following demo check the Allow Recursive Selection checkbox in the configurator):
TreeList / Server-side Selection
the Telerik team