From my own recent experience, most of the stations were already reached 10-20 minutes ahead of schedule, in spite of slowing down/waiting before the respective stations. From New Delhi to Mumbai:
1. Kota was reached 10 minutes ahead, and forced to wait till Dep time.
2. Nagda was reached 20 minutes ahead, and forced to wait till Dep time.
3. Vadodara was reached 10...
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minutes ahead, and forced to wait till Dep time.
4. Surat was reached 10 minutes ahead, and forced to wait till Dep time.
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So even without increasing speed/2 engines, etc, the TT can easily be shortened by 1 hour. But it looks like the Railways would "like" to speed it up and reach "ahead" of schedule, so as to maintain a good impression that the "train is never late".
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As for AK from Mumbai to NZM, the slack was even worse. In spite of reaching most of the stations almost 20 minutes ahead of schedule (and forced to depart at the right time), the train was waiting outside NZM/Faridabad for close to 1 hour!!!