Just ran into this. 30000-60000 isn't that weird. DUT IDs from a database for instance, that get autocompleted by the combobox.
Note that filling the strings of the combobox is NOT slow. It's filling the combobox with strings and values that is slow. .3 sec. for 3000, 27 sec. for 30000 items, this grows exponentially. 60000 takes 108 sec. When you stop the VI, the VI is 'reset', and that takes just as long.
The solution is to fill the combobox with just values. Then, when a string is selected, look that string up in the array of values.
Just don't select "edit items". It will update several times, and you'll be doing a lot of waiting. Closing with task manager and restarting is faster.