OK, 1.5 sec seems a long time to do anything. I'm surprised to hear that you can get a delay that long. I have never noticed anything like that - maybe because I have never looked. It appears that you are only sending a single character for data, so I guess any change in speed would be un-noticable. It may also be the reason why you appear to get a result using software serial at that speed. If that is indeed the case, don't expect to get a result when you send anything more than that. That speed is the kiss of death with software serial.
I also now understand that you are using software serial in a Mega, which is about the dumbest thing you can do in the Arduino world. I bet you haven't got a good reason to do that in the face of four hardware serial ports being available to you, and it is definitely not the sort of thing to show to your mother.
I suppose it is possible that the problem is at the Android end but, again, I have never noticed. There is one Android terminal that cannot log data. If the writer couldn't get that right, I imagine he would be the one that gives you high latency too, and it would be a good idea to replace it.