hmm, definitely seems like something is the matter. I just tested and on my i7-2600k, and I was able to playback a stream at 2x via RDP while also playing back a completely different camera stream in UI3 at the same time. Unfortunately, mine is so old, the Intel Graphics driver doesn't support the required WMI version to update in Task Manager.
- Baseline CPU (how I run day-to-day) :: UI3 only (JPEG HD), (Video update during Remote View default setting: 5sec) 14-18%
- UI3 (1 stream playback, 4MP cam, 1x speed, 720p UI3 window) 15-19%
- RDP + Live View (10fps) + UI3 view only = 45% peak, then 15-19%
- RDP + Live View (10fps) + UI3 Playback (4MP 1x speed) = 24% peak, then 18-24%
- RDP + Live View (10fps) + UI3 Playback (4MP 1x speed, 4k UI3 resolution) + Live playback (2MP, 2x speed)= 45% peak, then 36-48%
- RDP + UI3 view only + Remote Video refresh rate: Unlimited = 27% - 33%
Also these percentages might need to be taken with a grain of salt, as hyper-threaded processors do not increase linearly from 50% to 100%, beyond a certain percentage of CPU usage (~ 60-75% depending on load type, they will suddenly shoot up to 100%). My guess is some setting, or config item is putting you into that range and that's why you are slamming into the 100% bar.
Hitting a disk bandwidth limit, memory limit or a iGPU limit or something. Just a guess.