SQL massive performance difference using SELECT TOP x even when x is much higher than selected rows

I'm selecting some rows from a table valued function but have found an inexplicable massive performance difference by putting SELECT TOP in the query.

SELECT   col1, col2, col3 etc
FROM     dbo.some_table_function
WHERE    col1 = @parameter
--ORDER BY col1

is taking upwards of 5 or 6 mins to complete.

However

SELECT   TOP 6000 col1, col2, col3 etc
FROM     dbo.some_table_function
WHERE    col1 = @parameter
--ORDER BY col1

completes in about 4 or 5 seconds.

This wouldn't surprise me if the returned set of data were huge, but the particular query involved returns ~5000 rows out of 200,000.

So in both cases, the whole of the table is processed, as SQL Server continues to the end in search of 6000 rows which it will never get to. Why the massive difference then? Is this something to do with the way SQL Server allocates space in anticipation of the result set size (the TOP 6000 thereby giving it a low requirement which is more easily allocated in memory)? Has anyone else witnessed something like this?

Thanks