Runnable is excecuted long time after postDelayed's delay

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Mobile

Android

At some point I want my android background Service to execute something after 2 seconds. So - I had prepared a designated Handler:

private Handler handler;
...
HandlerThread thread = new HandlerThread("LNT");
thread.start();
handler = new Handler(thread.getLooper());

and then I call

handler.postDelayed (new Runnable(){
    public void run(){
        doSomething();
}, (long)(2000));

sometimes soSomething() is being called after 2 seconds, but many times it take up to 15 seconds !!

What am I doing wrong?
Is there another way to run something after a specific period of time?