Hi,
It can be due to :
1 - Codecs (you can install Vlc or MxPlayer for android. NOTE : if you have an ARM chip you have to instgall the ARM codec for MxPlayer)
2 - it can be your phone (Can't supprt hight resolution streaming)
3 - Your bandwith (Too slow for streaming HD)
4 - Many process started in your phone can slow down the stream operation
You can Give your phone model so I can Help you more in details and if possible you network config
See You