Run an application that creates a lot of threads, and press "Ctrl-C" to stop it.
e.g. try to "Ctrl-C" the following program:

class t{
  public static void main(String []args){
    try{
      for(int i=0; i< 800; i++){
        Thread t = new Thread(){
          public void run(){
            try{
              System.out.println(getName());
              sleep(100000);
            }catch(Exception e){}
          }
        };
        t.start();
      }
    }catch(Throwable t){
      t.printStackTrace();
// System.exit(1);
    }
    System.out.println("Press Ctrl-C please...");
  }
}

On my Linux system (Redhat 7.1 SMP on dual PIII-866), with Merlin b83,
Ctrl-C takes 29 seconds to finish!

Handling Ctrl-C should be quick.

Update:
It's reasonably fast using JDK-1.3.1 or 1.4-beta (by "reasonably fast",
I mean up to 5 seconds). 1.4-beta2 needs 20-30 seconds to finish Ctrl-C.