Playing big files with rosbag

Hi everybody, I am running Ubuntu 12.10, with ROS Groovy. My computer has a i7 processor with 6GB of RAM. I am recording in rosbag data from two kinects (and two skeletons). Of course, I din't recorded the "PointClouds", but only the raw images. However, my files go up to 50 GB for a 12minutes videos... My problem is that for such big files, "rosbag play" takes forever to start to play the data. I was wondering on the way it works. Indeed, I thought it played a stream of the data, thus, whatever the size of the file, it should not take that much longer to start playing. However, as I saw in my experiment, it is not the case. Thus I maybe thought that rosbag play is putting a lot in memory, or doing some "precomputation" ? If someone who knows how rosbag works could enlighten me of his knowledge, it would be great. I know this question is very technical, but I would really like to find a way to open my files after recording them, without having to buy a new "very powerful and expensive" computer... My first thought would be to record each topic in one bag, and then replay them all in the same time... But seems a bit stupid to do that, as I think rosplay would need to "realign" in time all the messages. Thanks in advance, Steph