Modify the CalcDifficulty
in Geth to return a static number, then rebuild Geth. Example:
func CalcDifficulty(config *ChainConfig, time, parentTime uint64, parentNumber, parentDiff *big.Int) *big.Int {
return big.NewInt(0x4000)
}
Source: AnswerAnswer to Is it possible to change the block target time?Is it possible to change the block target time?