DSM 6.1 Upgrade: DS214Play (slow since update)

Hello.

Since I updated DSM to 6.1, Plex (1.4.2) is running incredibly low on some vids (on TV, or browser, or anything else). Some work fine, and some are buffering every 5 seconds (CPU is near 100%).

Before the upgrade, there were no problem to play all my vids.

On Videostation, there's no problem, but I prefer to use Plex...

Any idea ?

Thanks in advance.