Modded server

Im currently running a 1.7.10 modded server off of a HP DL380 G7. It is running a less modded version of Crazy Craft. The server tends to lag, and can not keep up, I have allocated 18G of ram to the server, I would normally use 12 but sense the server is running off of ddr3 ram i decided to use a little more. The computer has dual zeon 3.3ghz processors, I have tried multiple things to try to get rid of the constant lag when loading new chunks or breaking blocks. I need some suggestions on what else i can do to fix this issue.