Valid parameter range reported by at#cpumode=? is #CPUMODE: (0-6) on a GE864-QUAD-V2 with Firmware 10.00.027
Documentation AT Command Reference v17 states:
0 - normal CPU clock @26Mhz
1 - CPU clock @52Mhz
2 - CPU clock @52Mhz, during GPRS TX/RX only
3 – CPU clock @104Mhz
4 - CPU clock @104Mhz, during GPRS TX/RX only
What does the coumodes 5 and 6 stand for?
A first look at the resulting performance depending on CPU mode is:
cpumode 0,2,5,6 take the same runtime, 100%, then cpumode 1 needs about 60% and cpumode 3 needs only 30% of that runtime.