Spring Transactional slows down complete process

I am trying to analyze a situation where I have two classes. One class is ProcessImpl which is starting point and internally calls other child transactions.I dont know whats going wrong. The processImpl is importing some stuff and writing related data to database.

Specs

Spring-orm version : 3.2.18.RELEASE.

JDK version : 1.8.

Db : H2 (on any db same performance is recorded).

Issue

If I remove @Transactional from ProcessImpl.processStage() the process takes ~ 50 seconds If I keep @Transactional from ProcessImpl.processStage() the process takes ~ 15 minutes. Dont know why this is happening. I have been trying to solve this issue since long but no luck. Please have a look at code below.

Requirement: The complete processStage() should complete or rollback completely, even if one of child transactions fail.

Fyi : I also get lot of messages like : "Participating in existing transaction". Tried to get over this by adding propagation=Propagation.NESTED to processStage() but did not work.

ProcessImpl Class.

public class ProcessImpl {

    /*This is the big transaction that calls other transactional stuff from MyServiceImpl
    * This is starting point you can say for the process...
    * 
    * If we remove @Transactional from here the process is lightning fast 
    * With transactional : 15minutes
    * Without transactional : 50 seconds
    * */
    @Transactional
    public void processStage(){
        MyServiceImpl mp = new MyServiceImpl();
        //do some stuff
        mp.doWork1();

        //do more work
        mp.doWork2();

    }

}

MyServiceImpl Class

class MyServiceImpl{

    @Transactional
    public void doWork1(){
        Object o = doChildWork();
        // and more stuff
        //calls other class services and dao layers
    }

    @Transactional
    public void doWork2(){
        //some stuff
        doChildWork2();
        doChildWork();
        //more work
    }

    @Transactional
    public Object doChildWork(){
        return new Object(); //hypothetical, I am returning list and other collection stuff
    }

    @Transactional
    public Object doChildWork2(){
        return new Object(); //hypothetical, I am returning list and other collection stuff
    }

}

Also, here will I get self invocation issue, which is not advisable in Transactional?