You need to apply those settings via the command line by telnetting or SSHing into the router. If you're not familiar with using the command line... well... I'm afraid that's beyond my ability to help with right at the moment, sorry.
that tells you exactly what to do (copy and paste the stuff into a specific location). You might need to tweak those until you find an ideal situation. Note that every time you modify that, you will need to reboot the router for the changes to take effect.
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Maximum Ports: 1024 (Default: 4096, Range: 256 - 4096)
TCP Timeout (in seconds): 300 (Default: 3600, Range: 1 - 86400)
UDP Timeout (in seconds): 120 (Default: 120, Range: 1 - 86400)
I went and changed the max ports to 2048, but I'll keep the other two as is until someone can tell me to change them
^^ ohai, I posted this way earlier in the thread, it's how to change the various timeouts that can't be changed through the GUI.
http://www.dd-wrt.com/wiki/index.php/Router_Slowdown#Script
that tells you exactly what to do (copy and paste the stuff into a specific location). You might need to tweak those until you find an ideal situation. Note that every time you modify that, you will need to reboot the router for the changes to take effect.