the phone limits the current going into it. nothing is going to change that. it is not the fault of anything. the phone has a very power hungry CPU. decoding/encoding a video ESPECIALLY an HD video, is going to chomp a lot of CPU cycles.
if you turn off wifi, bt, radio, etc, you may squeeze out a bit more play time, but not much.
even if you could send a hundred amps (!!) into the phone
, the circuitry in the battery will stop charging if it senses heat increasing beyond a set threshold--arresting the charging process until the temp cools--which it will have a while to wait because the CPU itself will generate its own heat.
how many videos are you trying to watch at one sitting on the phone?