I am binding a custom object list (custom collection) to a ComboBox
column inside a DataGridView to use as a typical lookup.

The list of items has up to 10,000 rows (8,000 currently) and when you
enter edit mode on the combobox cell you must wait up to 4 seconds to
see the dropdown of the list of items. I am handling the
EditingControlShowing and the EditingControl KeyDown events to
dropdown the ComBox when the user hits Enter. This wait also happens
if you click on the ComboBox dropdown arrow.

I understand that may be a lot of objects, but 4 seconds is an awfully
long pause for the user. I am looking for some ways to cut this down
to 1-2 seconds at least but still use the custom object binding.

The binding is done on Form load and the only thing I can think of is
that it is reloading the control every time the edit control is shown,
which is why it takes so long.

Does anyone know of a way to debug what is going on at a low level?

As well, if anyone has any options or has had this problem and figured
it out, please reply.

Thanks!