Transcoding lags but the server is not using all the CPU power?

Hi.

I have a file server/plex media server running Windows 2012R2 with Storage Spaces

When i transcode to my Samsung TV or Xbox 360 the playback "lags"/ it stoppes and buffers every few seconds. At one time the Samsung TV even displayed a error that the server doesn't have enough CPU.

But my server is only using around 25% when transcoding the rest is idle. is it not able to use all the cores/cpus?

Motherboard SuperMicro MBD-X8DAH+-F -B
CPU 2x Intel Xeon E5606, 2.13 GHz - Quad Core/1066/8 MB
RAM 6x Kingston DDR3 ECC Reg, 1333 MHz, DR x4, 4 GB