The HPO length locking servo is gain limited, and the error signal has coherence with down-stream sensors, including DARM.  Decreasing the ILS gain increases the coherence with DARM at several peaks above 200Hz (see attached).  We can't increase the gain much with the current servo shape (see 31121, servo board here), but a small gain increase reduces the coherence with DARM.

In the image: BLUE is low gain (servo gain 3.0 V), GREEN is nominal gain (servo gain 1.0 V), RED is max gain (servo gain 0.0 V)