Here's some code I Googled that will convert a YUV420 file to BufferedImage frames. You can use this as a base for what you want.
You probably won't be able to process 600 video frames a second on a PC either. You'll have to see how many video frames you can convert per second, and drop the rest of the frames.
Probably, the best way to process 20 video feeds is to have 20 threads grabbing a video frame, converting the video frame to a BufferedImage and passing the BufferedImage to the Event Dispatch Thread (EDT) for Swing to draw on the corresponding JPanel. When the thread comes back to grab the next video frame, you'll have automatically dropped the frames that the PC didn't have time to process.