Let's start from the second question. A row in a textbox cannot be "marked": you can detect which row the cursor is on by using ATTR_TEXT_SELECTION_START control attribute (but you will have to handle the case when the user selects more lines) and you can replace it prepending for example an asterisk (remember to delete it when you don't need it anymore).
On the listbox size, I suppose some improvement can be obtained by hiding the control while updating (be sure the listbox is not the active control and no screen refresh happens so that you don't see flickering).
Regarding the performance, while the text (item label) is always on screen you may consider that additional informations are not so crucial to maintain them inside the control all the time. That is, you could use the item value as the index into an in-memory object or a disk file where additional informations are stored: you can retrieve and display one single element when the user asks for it, leaving the control with a reduced amount of informations stored in it. It seems to me that this could make the control lighter and possibly reduce the slowering effect you are seeing now.
