This is a bit helpful in beginning to address lag and fix it:
http://www.spigotmc.org/wiki/lag-types-and-how-to-fix-them/
Also this:
http://www.sk89q.com/2013/03/improving-your-minecraft-servers-performance/
12 GB of ram is good, but equally important is your CPU. A common misconception is that RAM is everything, when really a good CPU is more important (Hosts will always try to sell you on RAM more than anything) There is lots of info on CPU out there, I highly suggest searching the Spigot forums (Rather than Bukkit or Minecraft forums).
Bending is also a very resource-intensive plugin. There is no real limit to the number of plugins you can have, it's really about how intensive those plugins are. It's best to try and minimize your # of plugins, i.e: Rather than have 3 different plugins to handle vanishing, fake leaving/joining, and other vanish-related tasks, try using VanishNoPacket, which has everything in one. It is also important to make sure that the plugins you use are optimized, which you can check by reading reviews (I highly suggest Spigot's resource page rather than the dying bukkit dev) and running Timings Tests (I believe timings works on Craftbukkit, but better on Spigot). For best results with timings, /timings on during a peak hour (When the most players are bending/joining/etc.) and let it run for the duration of that hour, at the end /timings paste and look at your results. If you don't know how to read timings, this will help:
http://www.spigotmc.org/threads/timings-guide-how-to-read-timings.2441/ . The test can help you see what plugins are using the most (Typically PK comes out on top, which make sense), as well as what world events are consuming too much memory, such as MobSpawn (Laggy spawner farms) and other Tick events (Some are indicative of redstone clocks and etcs.).
Also helpful is ClearLagg, which can run /lagg unloadchunks and /lagg gc. Unloading chunks and Garbage Collecting when things get laggy helps reduce the strain on the server w/o the need for a restart.
For addressing latency, it's best to try and see where your players are connecting from (Mainly the ones who complain the most about lag). There are GeoIP plugins for this, or whois searches, but if you don't want to do that you can always ask what county they're connecting from. The farther away they are from the host, the more likely they will get latency lag.
Hopefully this helps a bit, I remember struggling with the lag monster before lol. If you need more help, paste a timings here and I can take a look. Good luck!
