Query taking more time to execute due to lengthy column size
Even if all 6,000 rows are packed full of 4,000 Unicode characters (8,000 bytes), that's only 45 MB of data.  There's just no way it should take 10 minutes to read that much, even with a cold cache.

You're likely getting blocked by something, as Randi Vertongen mentioned in the comments.  The answer here isn't going to be indexing.  Run sp_WhoIsActive while your query is executing to see what's going on.  That should show you:


what the query is waiting on
if it's blocked (and what's blocking it)