Android app video (mp4) optimization

I have a completed app that is fully functioning and working. The app takes 8 png images and converts them into an mp4 video. The mp4 video is 480px wide in size and is only 3 seconds long, so it is a small video. However the file size is 2MB - 3MB, this is very very large and 200kb - 300kb can be achieved without loss of video quality. I was using a library called ffmpeg which is JS based and therefore very slow, ffmpeg was achieving 200kb - 300kb file sizes. So it is possible, however quickly outputting the video is important. The task for this project is very simple, reduce the file size of the videos my app outputs while maintaining the quality and video size (width/height). This should be a simple easy task for anyone proficient in the skills required. The app is a hybrid app, it is built with Cordova. But the plugin used to convert images to video is Java based. I can provide the app files and directions on where the Java code is in 1 to 1 discussions.