CheckedListBox was never meant as a substitute for tables, nor as a wrapper for ~1250 rows... As you've been suggested in comments, use a different control, like DataGridView
or maybe ListView
(if it makes sense in your scenario).
http://msdn.microsoft.com/en-us/library/system.windows.forms.datagridview.aspx
http://msdn.microsoft.com/en-us/library/system.windows.forms.listview.aspx