Child hierarchy template expand very slow

[Henry]

I'm using a Grid view with a custom hierarchy template. I bind it to my ViewModel and properties there.

The Custom template contains another Grid View.

The problem is that when expand the row and show related data it may cost more than 8 seconds. By the way there are 300 columns, 9 rows in child grid view. 
How can I solve it? And how can I load the child grid view together with parent grid view not when expand the child grid view?


Another issue is the child grid view can only show 250 columns and display the scroll bar.
How to show all the columns with no scroll bar?

Currently I am using DataTable in code behind as item source. Both main grid view and child grid view.

[Henry]

Any one can help me?