I concur what dawalker-1068762 has said. MERGE is a wonderful command but use carefully. If the source (SRC ) table is small and light it is feasable and nice to use. But if the table has millions of rows then you will hit a performance snag.
To get around this we are using MERGE along with a batch process. User very carefully! and design things in batches.
Wish MERGE had a parameter to do batch processing!