my c1200 work soo far soo well for me as compare to old Dlink Dusbin shaped router. which sucks like my grandma kancil
the only issue for me is the boot up time, quick annoying sometime when require a reboot , around 2 mins 35s
but after boot up it work brilliant!! even build it with vpn server function in c1200, i only can find this in high end router above rm600.

you can try lower down the high cpu frequency by trying the sugggestion below
1)turn off wps
2)off upnp
3)off ftp
4) turn off whataver function u can find in it

Don't turn off SPI Firewall...
And nothing to do with USB media sharing that can cause any problems...