Many new QNAP NAS include the ability for on the fly hardware accelerated transcoding.
My new NAS the HS-251 with 2gb of ram and a dual core 2.4ghz processor is unable to transcode all 1080p videos using plex to my iPad. If it does play it might take 20 seconds to load and then stop while transcoding catches up.
I have no problems with 720p content and some 1080p content works ok but still the time to start playing is quite long again I'm talking 10 - 20 seconds which isn't ideal.
If QNAP added support for hardware accelerated transcoding my NAS would be able to transcode 1080p content much better.
Page 10 of this QNAP PDF talks about the hardware accelerated transcoding capability. The NAS should be able to handle 1080p content just fine, Plex just need's to add support for it.
files.qnap.com/news/pressresource/datasheet/HD-video_HS-251_EN(web).pdf
The user experience of Plex on certain QNAP NAS models could be significantly improved with this feature.