I'm honestly wondering if it's just not a trick of the logic on the merge at this point.

I tried running a join between the output files and what would be the input to my MERGE statement. I ran an explain on that query and it ends up creating a BroadcastNestedLoopJoin. More times than not, nested loop joins have bedeviled my performance. I'm going to just try splitting the merge in to two separate calls and see if that does the trick for me.

It might just be that the explain on a MERGE doesn't show this because of how merges are executed.