1.
Mplayer loves to be CPU optimized (yes try building it from SlackBuilds.org ?)
2. I'm not familiar with Your xe303c12, but
sound modules can have tweaks (try # lsmod and # modinfo <sound module name>)
3. Playing a stream with lower resolution might help (I download with
$ youtube-dl -f 18 then play locally on slower platforms)
4. Usually it's the combination of
sound and video together spamming the (slow)
framebuffer and sound module - try run it without sound and compare CPU load (
gkrellm )?
5. There are
.asound.conf that can alleviate sound issues (down-sample, less resolution, disable re-sampling to/from
44100 (or
48000) (native sound HW bit-rate))
Usually checking all above would yield a solution?