The performance level of Astah depends on the RAM capacity of your machine.
Try following methods:

• Use the latest Java environment. (How to use a specific version of the JavaVM)
• Turn off the mouse cursor tracing option
  in some environments, having this option on will make rendering slower.
• Tune some garbage collection settings.
  When the value of INITIAL_HEAP_SIZE is set to 16m, the garbage collection does not work fully unless your computer uses more memory than 16MB.

NOTE: Do not set the value of MAXIMUM_HEAP_SIZE too small because it could cause "OutOfMemoryErrors" and Astah may not work properly.