I've done more evaluating, in hopes of gaining some clues about this mystery. Different files behave differently in MC and MPC-HC (some play smoothly in the former, some in the latter). Here's data on two files, the first of which won't play in MC without choking and sputtering, but does play in MPC-HC smoothly. The second file plays smoothly in both programs.
File 1:
Complete name: Samsung_SUHD_Picture_Quality Demo_Nano_Crystal Display_UK-Version.mp4
Format: hvc1
Codec ID: hvc1
Video ID: 3
Format: HEVC
Format/Info: High Efficiency Video Coding
Format profile: Main 10@L5.1@High
Codec ID: hvc1
Codec ID/Info: High Efficiency Video Coding
Bit rate: 50.0 Mbps
Maximum bit rate: 90.3 Mbps
Width: 3 840 pixels
Height: 2 160 pixels
Frame rate: 29.970 fps
Color space: YUV
Chroma subsampling: 4:2:0
Bit depth: 10 bits
Color range: Limited
Color primaries: BT.2020
Matrix coefficients: BT.2020 non-constant
File 2:
Complete name: 4K-TCL-H265.mp4
Format: MPEG-4
Format profile: Base Media / Version 2
Codec ID: mp42
Video ID: 2
Format: HEVC
Format/Info: High Efficiency Video Coding
Format profile: Main@L5.1@High
Codec ID: hvc1
Codec ID/Info: High Efficiency Video Coding
Bit rate: 46.4 Mbps
Width: 3 840 pixels
Height: 2 160 pixels
Frame rate: 29.970 fps
Standard: Component
Color space: YUV
Chroma subsampling: 4:2:0
Bit depth: 8 bits
Bits/(Pixel*Frame): 0.187
Writing library: x265 unknown:[Windows][MSVC 1700][64 bit]
Color range: Limited
Color primaries: BT.709
Transfer characteristics: BT.709
Matrix coefficients: BT.709
In all cases, the CPU is getting worked very hard, and the GPU is not getting worked very hard. But in the case of File 1, when playing back via MC, the CPU is maxing out and the GPU is practically at idle levels; when playing back via MPC-HC, the CPU is highly-used and the GPU is moderately used. For the relatively simpler File 2, when playing back via either program, the CPU is moderately-used and the GPU is lightly used.
I've tested with LAV filters set to CUVID HW Accel and DXVA2 (Native) HW Accel. Similar results.
Any ideas for things I should check or try, I'm eager to learn what might cause this and how to correct it. My understanding is that the typical results with a GTX960 playing back HEVC is that the GPU works at decoding and the CPU is hardly tasked. Not at all what is happening on my system...