Fortigate has iperf client for traffic testing built in, here are all the details

Starting with the FortiOS 5.x Fortinet have a built-in iperf3 client in Fortigate so we can load test connected lines. If new to iperf, please read more here
iperf in Fortigate comes with some limitations and quirks, so let's have a better look at them:
- The version of iperf used (in 5.x and 6.x firmware so far) is 3.0.9. This means it will not work with the iperf2 and its subversions.
- The tool can work as CLIENT only, i.e. it does not accept -s option. This means we can NOT run iperf test between 2 Fortigates, one of the peers has to be some Linux/Windows server with iperf3 -s running. It does NOT mean we can test only one direction, though - the command accepts -R option for reverse traffic.
- As you will see below, the command asks for Client and Server interfaces. The Server interface means on which Fortigate interface the remote server is located. The Client interface means ... I guess where the client is located. For wan testing I put it the same interface as the server-facing one.
- The tool accepts most of the command line options as a regular iperf3, except those mentioned already.
- In Fortigate with VDOMs enabled, iperf is available in the Global context only, i.e. outside of any VDOM.

So let's configure and run the test.
The default configuration is like shown below and it will run iperf test of the throughput between 2 interfaces of the Fortigate itself, not very interesting:

diagnose traffictest show Show the current configuration:

server-intf:    port1
client-intf:    port3
port:   162
proto:  TCP 

To run the test, let's set the port to 5201, protocol to TCP and client interface to port1 (Server interface is set to port1 by default already):

diagnose traffictest port 5201
diagnose traffictest proto 0 NOTE: 1 is for UDP and 0 is for TCP.
diagnose traffictest client-intf port1

We are ready to run the iperf test. On the remote server I have iperf3 -s running.
So, on the Fortigate I run:

diagnose traffictest run -c

Connecting to host, port 5201
[  5] local port 50670 connected to port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  24.8 MBytes   208 Mbits/sec  271   2.22 MBytes       
[  5]   1.00-2.00   sec  39.9 MBytes   335 Mbits/sec    5   1.03 MBytes       
[  5]   2.00-3.00   sec  14.7 MBytes   123 Mbits/sec  131    619 KBytes       
[  5]   3.00-4.00   sec  12.3 MBytes   103 Mbits/sec    1    594 KBytes       
[  5]   4.00-5.00   sec  8.02 MBytes  67.2 Mbits/sec    1    361 KBytes       
[  5]   5.00-6.00   sec  7.83 MBytes  65.7 Mbits/sec    0    385 KBytes       
[  5]   6.00-7.00   sec  7.83 MBytes  65.7 Mbits/sec    0    397 KBytes       
[  5]   7.00-8.00   sec  7.83 MBytes  65.7 Mbits/sec    0    403 KBytes       
[  5]   8.00-9.00   sec  7.83 MBytes  65.7 Mbits/sec    0    404 KBytes       
[  5]   9.00-10.00  sec  7.83 MBytes  65.7 Mbits/sec    0    419 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   139 MBytes   116 Mbits/sec  409             sender
[  5]   0.00-10.00  sec   137 MBytes   115 Mbits/sec                  receiver

Now let's run load test using UDP, and bandwidth of 50 Mb/sec.
dia traffic protocol 1 NOTE: this is not strictly needed if we use -u below, but why not ...
dia traffic run -c -u -b 50M

To see all the available options for Fortigate version of the iperf3, run:
dia traffictest run -h

FG1 # dia traffictest run -h
  -f, --format    [kmgKMG]  format to report: Kbits, Mbits, KBytes, MBytes
  -i, --interval  #         seconds between periodic bandwidth reports
  -F, --file name           xmit/recv the specified file
  -A, --affinity n/n,m      set CPU affinity
  -V, --verbose             more detailed output
  -J, --json                output in JSON format
  -d, --debug               emit debugging output
  -v, --version             show version information and quit
  -h, --help                show this message and quit
  -b, --bandwidth #[KMG][/#] target bandwidth in bits/sec (0 for unlimited)
                            (default 1 Mbit/sec for UDP, unlimited for TCP)
                            (optional slash and packet count for burst mode)
  -t, --time      #         time in seconds to transmit for (default 10 secs)
  -n, --bytes     #[KMG]    number of bytes to transmit (instead of -t)
  -k, --blockcount #[KMG]   number of blocks (packets) to transmit (instead of -t or -n)
  -l, --len       #[KMG]    length of buffer to read or write
                            (default 128 KB for TCP, 8 KB for UDP)
  -P, --parallel  #         number of parallel client streams to run
  -R, --reverse             run in reverse mode (server sends, client receives)
  -w, --window    #[KMG]    TCP window size (socket buffer size)
  -C, --linux-congestion <algo>  set TCP congestion control algorithm (Linux only)
  -M, --set-mss   #         set TCP maximum segment size (MTU - 40 bytes)
  -N, --nodelay             set TCP no delay, disabling Nagle's Algorithm
  -4, --version4            only use IPv4
  -6, --version6            only use IPv6
  -S, --tos N               set the IP 'type of service'
  -L, --flowlabel N         set the IPv6 flow label (only supported on Linux)
  -Z, --zerocopy            use a 'zero copy' method of sending data
  -O, --omit N              omit the first n seconds
  -T, --title str           prefix every output line with this string
  --get-server-output       get results from server

[KMG] indicates options that support a K/M/G suffix for kilo-, mega-, or giga-


Beware of CPU load the iperf tool can cause itself on Fortigate when testing - this is not usually a problem for newer models, but I've seen some quite large D series models reaching almost 100% on iperf test.