You can measure the network RTT by connecting to your hidden member (mongo04) and sending a ping to its sync source (mongo15).
Example:
PING 172.16.13.9 (172.16.13.9): 56 data bytes
64 bytes from 172.16.13.9: icmp_seq=0 ttl=64 time=0.882 ms
64 bytes from 172.16.13.9: icmp_seq=1 ttl=64 time=0.384 ms
64 bytes from 172.16.13.9: icmp_seq=2 ttl=64 time=0.570 ms
64 bytes from 172.16.13.9: icmp_seq=3 ttl=64 time=0.370 ms
64 bytes from 172.16.13.9: icmp_seq=4 ttl=64 time=0.502 ms
^C
--- 172.16.13.9 ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.370/0.542/0.882/0.186 ms
In the example above, the average RTT is 0.542 ms.
All the best,
– Rodrigo