Performance problem with AR-App on Android

Hello everyone. I'm currently working on an augmented reality app for a lecture in my university. For now it contains basically the video presentation via the WebCamTexture (which works fine since I use GetColor32 to avoid memory leak ;) and then I grab the Color information to Compute stuff.. I am aware that you cant help me to optimize these routines, but when I run the App on my Nexus 5 a strange thing happens: the first 2 seconds the app run perfectly, then the frame rate drops down significantly. (First Question: Why?) Now what is really silly, when I start tapping at the screen the performance gets great again. When I stop tapping, frame rate drops! So obviously the Device is capable to compute my stuff enough. Is there any way to get this speed without tapping??? Thank you very much!