Looking at the phone specifications for the Galaxy Mega phones, there are 3 different versions, all of which are likely to have difficulty with the Zmodo app, especially when streaming in HD. The phone only has 1.5 GB of RAM, and according to the phone’s specifications, has a maximum Android version of 4.4.3 if you have the Mega 2. If you have the oldest version of the Mega, the Android version tops out at 4.2.2.
Given the resource intensive nature of the Zmodo app, phones with less than 2GB of RAM are likely to have significant issues with slow loading, freezes and crashes. This is made worse on phones running outdated Android versions.
One step that can help the app function as well as possible would be to manually close out other apps running in the background. This will make more RAM available for the Zmodo app to use.
The new platform of the Zmodo app will remember the viewing quality that you were last using, and open the video stream at that quality. I would recommend ensuring that you only use the LD option with that phone.