[mythtv] mythtranscode performance

[Robert Tsai]

Recently (maybe within the past week) mythtranscode performance has
dropped from requiring approximately 1x time to now requiring 2x time
(e.g., now requires 2 hours to transcode a 1-hour recording). I
haven't changed any of my transcoding parameters in the meantime.
Commercial flagging has not seen a similar drop (e.g., commflagging
time has remained the same). 

Has anyone else noticed this, or would anyone suspect a particular
module? Would the recent XvMC merge commits affect transcoding
performance (I noticed it has changed some of the avformat decoding
code, but I haven't studied the diff yet)?

--Rob