Strange elbow behind tub spout

[H. Lee Taylor]

In order to improve water pressure I removed the tub spout and the 3.5 inch nipple behind it. Cleaned up spout threads and threads of the interior elbow (which were not terribly encrusted), installed new nipple: no improvement. With nipple out, examined elbow with flashlight which has what appears to be a built-in restrictor at the end of the threads, circular, with 1/4 " hole in the center and at the bottom an oval hole (same diameter) as if someone had taken a bite out of a donut. I tried to "unscrew" it (gently) but it appears to to be cast into what looks like a brass elbow. Could it be the cause of the lower pressure? Why now--it's been there for years? The system includes separate faucets for hot and cold and a diverter on the spout that operates the shower. Adjacent vanity has great pressure. Why only the tub (and shower)?