Even though this topic is already some months old I would like to share a little addition as I have been struggling with piCore 7.0 video playback on RPi2 lately.
The software decoder will of course still fail playing back 1080p video files, but lower resolutions will run smoothly. Only issue I ran into - and it took me quite a while to realise - was display resolution. ffplay worked with up to 540p in fullscreen on a FHD display (1920x1080), but 720p and fullscreen scaling was visibly lagging (audio still fine).
Normally all modern displays (computer monitors or TVs) support HD resolution (1280x720) so it is a pretty simple solution to lower the RPi's output resolution by editing config.txt:
hdmi_group=1
hdmi_mode=4
Save, reboot and 720p video files will play fine in fullscreen as no upscaling is required.