tcpdump показывает что все эти 23 сек nginx активно болтает с apache
По всей видимости на nginx какой то параметр зажат.

15:00:30.494990 IP nx.40281 > ap.http: tcp 0
15:00:30.495110 IP ap.http > nx.40281: tcp 0
15:00:30.495210 IP nx.40281 > ap.http: tcp 0
15:00:30.495372 IP nx.40281 > ap.http: tcp 554
15:00:30.499088 IP ap.http > nx.40281: tcp 1448
15:00:30.499097 IP ap.http > nx.40281: tcp 1448
15:00:30.499099 IP ap.http > nx.40281: tcp 1448
15:00:30.499102 IP ap.http > nx.40281: tcp 1448
15:00:30.499104 IP ap.http > nx.40281: tcp 1448
15:00:30.499106 IP ap.http > nx.40281: tcp 1448
15:00:30.499108 IP ap.http > nx.40281: tcp 1448
15:00:30.499110 IP ap.http > nx.40281: tcp 1448
15:00:30.499112 IP ap.http > nx.40281: tcp 1448
15:00:30.500569 IP nx.40281 > ap.http: tcp 0
15:00:30.500593 IP ap.http > nx.40281: tcp 1448
15:00:30.500597 IP ap.http > nx.40281: tcp 1448
15:00:30.501191 IP nx.40281 > ap.http: tcp 0
15:00:30.501206 IP ap.http > nx.40281: tcp 1448
....
15:00:55.464914 IP ap.http > nx.40281: tcp 479
15:00:55.465402 IP nx.40281 > ap.http: tcp 0
15:00:55.465478 IP nx.40281 > ap.http: tcp 0
15:00:55.901995 IP ap.http > nx.40281: tcp 1448
15:00:55.902466 IP nx.40281 > ap.http: tcp 0
15:00:55.902868 IP nx.40281 > ap.http: tcp 0
15:00:55.902886 IP ap.http > nx.40281: tcp 0