Quick way to change MP4 to AVI?

I notice that HD movie files in MP4 format stutter and run choppy on the WD Media Player.

But HD movies in either AVI or M2TS run smoothly on the WD Media Player.

However converting a full movie from MP4 to AVI takes well over an hour.

Is there any quicker way to do this? I know there’s a way to merge video files in to MKV files that only takes a few minutes - the problem is that MKV doesn’t work on the WD Media Player. Does WD plan on fixing this problem with playing larger HD MP4 files?

Please tell us what model WD TV player you are using (Live, Plus, Hub, or one of the older models?) and what firmware is currently installed.

http://community.wdc.com/t5/General-Discussions/Noticed-that-HD-MP4-files-play-choppy-on-the-WD-Media-Player/m-p/121086/highlight/true#M18225

Its a WD TV HD (Gen2) player with the latest firmware.