Server Performance Issues - Details Included

So since I got my server about a month ago, I've been having some performance issues. I've narrowed down the possible causes, but I'd like the opinions of people perhaps more knowledgeable than I on the subject

The Relevant Server Specs:
Linux CentOS
4 cores running 2.5Ghz each (I've found out that this kinda sucks since MC and java are not capable of multithreading, unless someone can correct me)
8GB RAM (unknown speed, need to check that, rented server)

Command Line:
taskset -pc 1 java -server -Xmx8192M -Xincgc -jar craftbukkit.jar
(I use taskset to force it to use one processor because if it uses all it still only uses a total of 100% and all the jumping around causes processing lag)

Server Plugins:
FalseBookCart
FalseBookIC
GroupManager
FalseBookBlock
SignColours
FalseBookExtra
HelpMessage
Lockette
KiwiAdmin
Permissions
RepairChest
HeroicDeath
Residence
BorderGuard
WorldEdit
iConomy
FalseBookCore
Essentials
LocalShops
TheDeadlies
EssentialsSpawn
EssentialsChat
Jobs
Administrate

The Problem:
We don't have much block lag for the most part, but there is a lot of time lag. Stars jump backward often and even during the day the sun is sometimes glitchy. PvP and PvE combat is also lagged because of this. When it hits 30 players it's getting to be unplayable...and this server has twice the power of the last one I was on which ran 50 players with little lag. I read up on the "Increasing Server Performance" thread but I'm not sure what to make of a lot of the stuff in it. Would anyone be kind enough to lend me a hand? Pulling my hair out over this one.

 

Well, with the amount of plugins, I'm not surprised. LAg is only to be expected, especially with memory muchers like Worldedit on there. Also you said you have 30 players? Well, a good rule of thumb is 1GB Ram for every 4-6 players. So, running with so many plugins, I'd say you're running damn close to maxing out your server.
You could also try setting a mimimum server usage into your startup code by replacing the xingc with Xms6140M