Some suggestions that I hope help:

java -version to see what version you are running. (May help with those on Mac OS X also that are having issues.)

If you are not running 1.6.11+, upgrade. There were major Java changes from 1.6.9 - 1.6.11 and you want to be on 1.6.11 or later.

This () is a good tutorial for Ubuntu/Debian people. If you are only running 32-bit, just do follow those instructions. Most distributions tend to be running an older version for stability purposes.

If you are running 1.6.11+ and are having slowness issues like you mentioned, the JRE limits the amount of memory a single application can use. There are two parameters you may want to include in your command. -Xms is the initial memory allocated for the application and -Xmx is the maximum memory allocated.

java -Xms128m -Xmx512m -jar Terrainlinx_Planner.jar
will set the initial amount of memory used to 128 MB and the maximum to 512 MB. These are probably decent default settings for the application. If you notice it getting slower when you start building even larger layouts, increase the maximum.