Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off QoS options.
Turn off Advanced DNS Services if you have this option.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices on the router.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.
What wireless modes are you using?
Try single N?
What security mode Preferred is WPA-Personal. Auto TPIK and AES.
Also be aware that the router is processing connections while connected to the router, wired and wireless. If you really want to pick up speed, I would install and external router, this will separate unneccessary router processing from the devices and make it so that your devices connect with each other with out router interference. I recommend upgrading devices to Gb hardware and using Cat6 to achieve greater speeds.