Note -i's default in the man page.

src/iperf3.1

@@ -26,7 +26,8 @@ set server port to listen on/connect to to \fIn\fR (default 5201)
 [kmKM]   format to report: Kbits, Mbits, KBytes, MBytes
 .TP
 .BR -i ", " --interval " \fIn\fR"
-pause \fIn\fR seconds between periodic bandwidth reports
+pause \fIn\fR seconds between periodic bandwidth reports;
+default is 1, use 0 to disable
 .TP
 .BR -F ", " --file " \fIname\fR"
 client-side: read from the file and write to the network, instead