check to make sure you are on the latest firmware available which is 220.127.116.11. Another good thing to confirm is the wireless channel that you are using and making sure it is not heavily used in your area. You can do an inssider scan to see which channels are available and if all channels are in use, you can choose the one with the least use.
keep in mind when moving from a consumer router to a business grade router you have higher security and more features. you can always test disabling SPI in the firewall settings.
Latest firmware is installed. I've also scanned nearby SSIDs and picked a channel not used by anyone close. Also ensured that WMM was disabled on the wireless settings (something I read elsewhere could help).
I couldn't find an SPI referenced in the firewall settings, but I did find an SIP ALG setting enabled in advanced firewall settings. Disabling it seems to have improved download speed to ~20 Mbs peak.
I should add a few things -- I recently updated my max speed from the provider from 25 to 50Mbs advertised (no change in hardware, just updated the package).
I instantly got a boost to 57Mbps when direct-wired to either router. I didn't test the WiFi speed on the old router after the package upgrade, but prior to upping the package, I got ~23 wirelessly on the G router on the 25 Mbs package, if that makes sense.
The other thing to add is this: If in your opinion the slower wireless speed is a tradeoff for the added security of the firewall, then it might just be worth it. :-)
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