Replicating a simple match function in query

= Table.AddColumn(Source, "Custom", each if List.Contains(Table.Column(Source, "Col A"), [Col B]) then 1 else 0)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyVtJRMjYyBJKGSrE60VC2kSGINACLGIFFTAyNICKxAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Col A" = _t, #"Col B" = _t, Result = _t]),
    #"Added Custom" = Table.AddColumn(Source, "Custom", each if List.Contains(Table.Column(Source, "Col A"), [Col B]) then 1 else 0),
    #"Changed Type" = Table.TransformColumnTypes(#"Added Custom",{{"Col A", Int64.Type}, {"Col B", Int64.Type}, {"Result", Int64.Type}, {"Custom", Int64.Type}})
in
    #"Changed Type"

Go to Advanced Editor.

Code Before:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyVtJRMjYyBJKGSrE60VC2kSGINACLGIFFTAyNICKxAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Col A" = _t, #"Col B" = _t, Result = _t]),
    #"Added Custom" = Table.AddColumn(Source, "Custom", each if List.Contains(Table.Column(Source, "Col A"), [Col B]) then 1 else 0)
in
    #"Added Custom"

Change the #Added Custom step to:

= let
buffer_list = Table.Column(Source, "Col A"),
#"StepOne" = Table.AddColumn(Source, "Custom", each if List.Contains(buffer_list, [Col B]) then 1 else 0)
in #"StepOne"

Code After:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyVtJRMjYyBJKGSrE60VC2kSGINACLGIFFTAyNICKxAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Col A" = _t, #"Col B" = _t, Result = _t]),
    #"Added Custom" = 
        let
            buffer_list = Table.Column(Source, "Col A"),
            #"StepOne" = Table.AddColumn(Source, "Custom", each if List.Contains(buffer_list, [Col B]) then 1 else 0)
        in #"StepOne"
in
    #"Added Custom"

List.Buffer will work if you want to create your own list with values.

Doc: https://learn.microsoft.com/en-us/powerquery-m/list-buffer

In this case we are using a variable buffer_list that holds all values from Col A and then we are reffering to that variable, without creating it for every row. It will work faster.