Best solution (performance) to get list with unique values from two table columns

I have a table with 4 columns and 200 rows. There is a From_CoO and a To_CoO column. I want to get a list ('ListWithCoOs') of the unique values from both columns. E.g.:

- From_CoO has values: MEX, NLD, MEX, null, null, null

- To_CoO has values: SPA, NLD, null, NLD

then the result should be:

- ListWithCoOs: MEX, NLD, null [yes, even this one!], SPA

First I solved this with the following code:

//Get from each column unique values, combine unique values and get unique values of combined list
TestInput = Table.Buffer(Source{[Name="TestInput"]}[Content]),
ListWithCoOs = List.Distinct(List.Combine({List.Distinct(TestInput[From_CoO]), List.Distinct(TestInput[To_CoO])})),

However performance was bad. So I changed it into the following:

TestInput = Table.Buffer(Source{[Name="TestInput"]}[Content]),
#"Removed Duplicates" = Table.Distinct(TestInput, {"From_CoO"}),
listFrom = List.Distinct(#"Removed Duplicates"[From_CoO]),
#"Removed Duplicates2" = Table.Distinct(TestInput, {"To_CoO"}),
listTo = List.Distinct(#"Removed Duplicates"[To_CoO]),
ListWithCoOs = List.Distinct(List.Combine({listFrom, listTo})),

This performs much better! But why? It are only 200 records. When in the Power Query user interface (Power Query Editor) I click the button on top of the column, in no time the unique values are shown.