/buildcraft/config/buildcraft.cfg:

    Change network.updateFactor=10 to network.updateFactor=20
    Can be set higher for even less lag.
    Decreases the block updates of BuildCraft.

/config/ModularForceFieldSystem.cfg:

    Change maxDeflectorDistance=10 to maxDeflectorDistance=5
        Can be set lower for even less lag
        This decreases block updates caused by Deflector force fields
    Change maxProjectorSize=32 to maxProjectorSize=16
        Can be set lower for even less lag
        This decreases block updates caused by Projector force fields
    Change maxTubeRadius=5 to maxTubeRadius=3
        Can be set lower for even less lag
        This decreases block updates caused by Tube force fields
    Change refreshSpeed=100 to refreshSpeed=1000
        Can be set higher for even less lag
        This decreases block updates caused by force fields

    /redpower/redpower.cfg:
        Change blockbreaker.blacklist="" toblockbreaker.blacklist="4,49,128,179,194,214"
            This disables a few dupes and EMC farms..
        Change deployer.blacklist="" to deployer.blacklist="179,213,214,6362"
            This patches an exploit that can be used to crash your server, as well as a few dupes.
        Change whenfull="drop" to whenfull="vanish"
            This causes items to be destroyed instead of dropped which would cause your server to lag.

    /bukkit.yml:
        Change monsters: 70 to monsters: 35
            Can be set lower for even less lag
            This makes less monsters spawn per chunk and will lag your server less.
        Change animals: 15 to animals: 10
            Can be set lower for even less lag
            This makes less animals spawn per chunk and will lag your server less.
        Change water-animals: 5 to water-animals: 2
            Can be set lower for even less lag
            This makes less squids spawn per chunk and will lag your server less.
        Change animal-spawns: 400 to animal-spawns: 1000
            Can be set higher for even less lag
            This makes animals wait to spawn and will lag your server less.
        Change monster-spawns: 1 to monster-spawns: 250
            Can be set higher for even less lag
            This makes monsters wait to spawn and will lag your server less.