Like i said 99% of laggy playback on them is the lack of HW acceleration, beyondxbmc uses some type of lib file to get round this so even 1080p runs flawless, I have a similar set up to the post above me running everything through mxplayer but beware dont mess too much as if you're a live tv streamer some of the streams wont play using the above, easy fix though just edit the config file to use default player on rtmp (iirc its those type) streams.
mines a rockchip dual core box (not stick) and is zippy as hell and loads everything fast and plays all files inc 1080p faultless, I ve been through the shite and searched and searched until i got it working bang on and the beyondxbmc is by far the best ive used.
Take that advice or leave it its makes no difference to me but if you're struggling with slow shitty boxes and laggy playback do it.. cpu runs at around 35/40 whereas frodo 12 was running 100% virtually all the time.