100% CPU is fairly normal, especially as you add stuff which the various daemons may then go ahead and catalogue and add to the various databases etc. I think my MCM has basically spent most of the past year and a half or so running like that, and it works fine.
Mine plays fine via ethernet and NFS to my Raspberry Pi running Kodi and feeding into my TV. If you’re streaming over the internet though, or locally via wifi then you’ll see speed degradations and glitching or buffering sometimes if your links aren’t quick enough.
But the simplest rule is to go through the options on the dashboard and turn off services and things that you don’t want or aren’t using. Here I include things like AirPlay, DLNA, cloud access etc if those are not used on your network or devices (similar to what Iluna_Polanco was referring to above). Two CPU hitters are the latter two, as both involve indexing and database tracking.
The version of Twonkymedia that ships with the current Mycloud firmware is old and has some bugs.
Each time I restart my MyCloudMirror, it hangs whilst creating thumbnails on a MP2 VOB file, it’s always the same file, and always hangs. I have left it weeks pegged at 100% CPU. To sort it out, I have to kill the thumbnailing task and then Twonkymedia moves onto the next file, and eventually finishes it’s startup scan, and the CPU drops to it’s nominal 10%
There’s a new version of the firmware (OS3) due out in a couple of weeks or so - see the press release here.
Hopefully that should clear up a few of the lingering bugs and outdated elements, and generally make a few long-requested improvements to various aspects of operations.