How can someone alone laggy a whole GOOD server?

Listen, I have a i5 with 8gb ram running my minecraft server and bukkit and 69 plugins. Ive been the whole week trying to find a unknown cause of extreme lag (less than 4 TPS).

CPU and disk access are 80% idle; Ten people online; No packets any kind of network attack... It is latest java 7 64bits and Centos 64.

I was loosing my hope and then I commented in chat: "I just can't find the lag cause... " All of sudden a player poped out and said "It's me causing it" and surprisingly it indeed was him.

What is happening here? I have several anti cheat plugins and I do a good job fighting it, but this kind of issue left myself hopeless... How can he affect the server is a way that TPS go down to 4???

Please ask any data you wish. I find it a really serious stuff, If players can render a server useless that easy...

Can someone please help me? Is there anything even possible to avoid it? How can it be even possible? Do the server wait for anything from the clients? Please advice...

 

Could we get your plugin list? (Code or spoiler list is preferred to safe us space)
To save YOU time in writing it out, you can get it with "pl" (Without the quotation marks) in the console, highlight it, and paste it... (I'm implying you're doing the boots and such via Putty)

 

Moved to the correct forum.

 

Thanks for the quick response. I am a 37 year old guy and I am a ISP admin, so please go streight to the point... Here is the data you asked, need versions too? I try to keep all of them up to date as possible I can.

I guess youre suspecting of some backdoor or faulty code on plugins? Ive disabled most of them, but not all, trying to find out how they can drop down the TPS so badly with no success at all.

Plugins I have on all the time:

Code:

21:27:42 [INFO] Plugins (69): Orebfuscator, obuShutTheHellUp, GroupManager, WallClock, FatGiants, mcjobs, NormalizedDrops, Vault, Multiverse-Core, PvPToggle, MyHome, Fizzled, AfkManager, TreeAssist, TopPVP, RemoteToolkitPlugin, AutoMessage, PerformanceMonitor, Spectate, Dogtags, SurvivalGames, AntiAdvertising, Permissions, BanAnnouncer, Curseban, SignLogger, Votifier, CommandBlocker, KillsPay, ChunkFixer, Citizens, LagMeter, SBC, PlgLogCmd, NoCheatPlus, WorldEdit, NoSwear, PermissionsBukkit, Remover, SimpleSort, Stargate, LogBlock, CombatTag, WorldGuard, MobileAdmin, Appleseed, Punishmental, TreasureHunt, iConomy, Cannons, WorldSize, CraftBookCommon, KillBox, Reservations, CommandLimiter, N3W_TheEndAgain, Essentials, CreeperHeal, CraftBookMechanisms, DynamicEconomy, dynmap, EssentialsChat, DeathTpPlus, EssentialsProtect, EssentialsSpawn, EssentialsGeoIP, Monster Apocalypse, Factions, WorldBorder

Plugins I have disabled once (altogether) to see if help to solve the issue:

Code:

98:76:54 [INFO] Plugins (32): obuShutTheHellUp, WallClock, FatGiants, mcjobs, NormalizedDrops, PvPToggle, Fizzled, AfkManager, TreeAssist, TopPVP, Spectate, Dogtags, SurvivalGames, BanAnnouncer, KillsPay, ChunkFixer, Citizens, NoSwear, Remover, SimpleSort, Stargate, LogBlock, CombatTag, Punishmental, TreasureHunt, Cannons, WorldSize, KillBox, Reservations, dynmap, DeathTpPlus, Monster Apocalypse 

I was asking the person who did it and he couldn't explain with real facts to repeat the issue. So, I am guessing it was unintentional (maybe) since it is all good. The only thing I dont get is how a client has so much power of my server (to drop it down to 4 TPS) while I see a avg 10 packets per second on its connection. I am trying to get more information about it...

EDIT by Moderator: merged posts, please use the edit button instead of double posting.