regarding Frontend Playback Groups
None of those settings should have any affect on playback performance. All recordings (LiveTV or scheduled) get recorded straight to disk. Because of the tight fsync loop in mythbackend, they will hit the disk within a second of them being captured. There is no setting that will change this behavior, and do to issues with the standard IO scheduler in Linux, it is not something that can or should be removed.
In Linux, all disk read and writes are cached to memory. They remain there indefinitely, until there either some other process needs that memory, or there is new data that needs to be cached. That means as you write a recording to disk, Linux effectively creates a ringbuffer for us, and as long as you are sufficiently close to live that the video has not been flushed out of the cache, you will not have to re-read the video from the hard drive. This cache constitutes all free memory not currently being used for applications, so unless you are trying to run a combined machine on under 512MB of memory, this will far exceed anything you may see on the embedded cache in a hard drive.
Even should you have to re-read from the disk, the highest bitrate content you may be recording with MythTV will be around 2.5MB/s. Even combining simultaneous read and write, plus database access, this is a trivial amount for any hard drive modern enough to be using SATA 3.0. You would only start running if you were doing a dozen recordings simultaneously, had absolutely awful drive fragmentation, or were otherwise causing very heavy load on the disk using an external application.