Checking performance of video-editing software

If I use a program to transcode AVCHD or whatever to e.g. ProRes then what load do you expect to see on the GPU or the video memory controller using GPU-Z? For me it's like <15% and I wonder whether it's because it uses only a certain part of the GPU or it's just bad software.

i7 6700, GTX 950

It's possible your AVCHD decoding is done on GPU, but the encoding to ProRes is probably CPU.

Okay but CPU is also always <25%. There are no cores a 100% so I figure something else is causing the bottleneck?

Disk speed perhaps? Post detailed system specs.

OS: Win 10 Pro 64-bit
Mainboard: ASUS Z170 Pro Gaming, board revision 1.04, BIOS v1904
CPU: Intel i7 6700, Stock HSF, Undervolt Offset -0.15 V, LLC 4
RAM: Corsair LPX Vengeance 2x8GB DDR4-3000 (CMK16GX4M2B3000C15)
Graphics: Gigabyte NVIDIA GeForce GTX 950 w/ 2 GB
Sys Drive: Samsung Evo 750 SSD, 250 GB
Data Drives: OCZ Vertex 2 SSD, 60 GB; WDC WD1002FAEX-00Y9A0, ~1 TB; WDC WD2003FZEX-00Z4SA0, ~2 TB; WDC WD2002FAEX-00MJRA0, ~2 TB; RAID 0, ~300 GB [Maxtor 6B160MO x 2]
PCI-E SATA expansion: IOCREST SATA III 4-port PCI-e (SI-PEX40064)
PSU: XFX TS 750W (P1-750S-NLB9)

I don't think it's the drive which is: WDC WD1002FAEX-00Y9A0, ~1 TB with benchmark (MB/s):

Sequential
Read 140
Write 125
Mixed 121

Random
4K Read 1.29
4K Write 2.53
4K Mixed 0.36

If I use a program to transcode AVCHD or whatever to e.g. ProRes then what load do you expect to see on the GPU or the video memory controller using GPU-Z? For me it's like <15% and I wonder whether it's because it uses only a certain part of the GPU or it's just bad software.

i7 6700, GTX 950

Who said your program is using GPU at all?
It probably doesn't touch it or 15% is already enough to give decoding speed which saturates encoding part which is done on CPU.
If your CPU is low then your software is crap. 25% sounds like your encoding is single threaded. Is it ffmpeg based and uses prores_ks option? ProRes (Kostya) use to be single thread in ffmpeg. Very recently it has changed and now it's much faster.