Code

package com.modcrafting.mobspeed;

import java.io.File;
import java.io.IOException;
import java.lang.reflect.Field;
import net.minecraft.server.EntityHuman;
import net.minecraft.server.EntityLiving;
import net.minecraft.server.EntityVillager;
import net.minecraft.server.EntityZombie;
import net.minecraft.server.PathfinderGoalBreakDoor;
import net.minecraft.server.PathfinderGoalFloat;
import net.minecraft.server.PathfinderGoalLookAtPlayer;
import net.minecraft.server.PathfinderGoalMeleeAttack;
import net.minecraft.server.PathfinderGoalMoveThroughVillage;
import net.minecraft.server.PathfinderGoalMoveTowardsRestriction;
import net.minecraft.server.PathfinderGoalRandomLookaround;
import net.minecraft.server.PathfinderGoalRandomStroll;
import net.minecraft.server.PathfinderGoalSelector;
import net.minecraft.server.WorldServer;
import org.bukkit.Server;
import org.bukkit.configuration.file.FileConfiguration;
import org.bukkit.craftbukkit.CraftWorld;
import org.bukkit.craftbukkit.entity.CraftZombie;
import org.bukkit.entity.LivingEntity;
import org.bukkit.entity.Zombie;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.CreatureSpawnEvent;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

public class Mobspeed extends JavaPlugin
  implements Listener
{
  public void onDisable()
  {
    reloadConfig();
  }
  public void onEnable() {
    getDataFolder().mkdir();
    File cfg = new File(getDataFolder(), "config.yml");
    if (!cfg.exists()) {
      try {
        cfg.createNewFile();
      } catch (IOException e) {
        e.printStackTrace();
      }
      getConfig().set("ZombieSpeed", "0.23F");
      saveConfig();
    }
    getServer().getPluginManager().registerEvents(this, this);
  }
  @EventHandler
  public void onCreatureSpawn(CreatureSpawnEvent event) {
    if (!(event.getEntity() instanceof Zombie)) return;
    EntityZombie zombie = ((CraftZombie)event.getEntity()).getHandle();
    try
    {
      Field fGoalSelector = EntityLiving.class.getDeclaredField("goalSelector");
      fGoalSelector.setAccessible(true);
      Float speed = Float.valueOf(0.23F);
      if (getConfig().getString("ZombieSpeed") != null)
        speed = Float.valueOf(Float.parseFloat(getConfig().getString("ZombieSpeed")));
      PathfinderGoalSelector gs = new PathfinderGoalSelector(
        (((CraftWorld)event.getEntity().getWorld()).getHandle() != null) &&
        (((CraftWorld)event.getEntity().getWorld()).getHandle().methodProfiler != null) ?
        ((CraftWorld)event.getEntity().getWorld()).getHandle().methodProfiler : null);
      gs.a(0, new PathfinderGoalFloat(zombie));
      gs.a(1, new PathfinderGoalBreakDoor(zombie));
      gs.a(2, new PathfinderGoalMeleeAttack(zombie, EntityHuman.class, speed.floatValue(), false));
      gs.a(3, new PathfinderGoalMeleeAttack(zombie, EntityVillager.class, speed.floatValue(), true));
      gs.a(4, new PathfinderGoalMoveTowardsRestriction(zombie, speed.floatValue()));
      gs.a(5, new PathfinderGoalMoveThroughVillage(zombie, speed.floatValue(), false));
      gs.a(6, new PathfinderGoalRandomStroll(zombie, speed.floatValue()));
      gs.a(7, new PathfinderGoalLookAtPlayer(zombie, EntityHuman.class, 15.0F));
      gs.a(7, new PathfinderGoalRandomLookaround(zombie));
      fGoalSelector.set(zombie, gs);
    } catch (NumberFormatException e) {
      e.printStackTrace();
    } catch (SecurityException e) {
      e.printStackTrace();
    } catch (NoSuchFieldException e) {
      e.printStackTrace();
    } catch (IllegalArgumentException e) {
      e.printStackTrace();
    } catch (IllegalAccessException e) {
      e.printStackTrace();
    }
  }
}