UNION ALL vs OR condition in sql server query

I have to select some rows based on a not exists condition on a table. If I use a union all as below, it gets executed in less than 1 second.

SELECT 1 FROM dummyTable
WHERE NOT EXISTS
(

SELECT 1 FROM TABLE t
WHERE Data1 = t.Col1 AND Data2=t.Col2

UNION ALL

SELECT 1 FROM TABLE t
WHERE Data1 = t.Col2 AND Data2=t.Col1

)

but if I use an OR condition, it takes close to a minute as SQL server is doing a table lazy pool. Can someone explain it?

SELECT 1 FROM dummyTable
WHERE NOT EXISTS
(

SELECT 1 FROM TABLE t
WHERE ( (Data1 = t.Col1 AND Data2=t.Col2) OR (Data1 = t.Col2 AND Data2=t.Col1))
)