Hi, I'm having trouble capturing my desktop screen using QuickTime. I need to create short 3 min video tutorials. I've followed all of the steps necessary; I've opened QuickTime, selected 'New Screen Recording', and clicked 'Built In Microphone' (with my iPhone earphones connected), and then selected the area on the screen to record. All appears ok, but when I play back the video, the first 5 seconds are silent, the video is extremely laggy and it cuts short. I'm wondering if my iMac may be the problem? It's 8 years old and is extremely slow anyway. Would this be the cause?