Solution 1:

Yes, IPERF does exist for DD-WRT routers. I have never used it myself, but I can point you to this Web page of the official DD-WRT online manual, which discusses it along with a number of equivalent tools. However, the link it provides to download IPerf is no longer operational (just try it!), so I am giving you an alternative link where you are told where to get it: from Pyramid Linux, a distro (Google it) that you can download from here. You can simply copy the binary to your DD-WRT router, like carla Schroeder suggests in the second reference, and you should be all set.

Should this fail for any reason (you my have a DD-WRT build different from mine), go back to reference 1, where you will find other alternatives discussed.

The code for IPERF has moved, it can be found here, while the older version is here.