Constant common_runtime::consts::MILLISECS_PER_BLOCK

pub const MILLISECS_PER_BLOCK: u64 = 12000;

This determines the average expected block time that we are targeting. Blocks will be produced at a minimum duration defined by SLOT_DURATION. SLOT_DURATION is picked up by pallet_timestamp which is in turn picked up by pallet_aura to implement fn slot_duration().

Change this to adjust the block time.