GIMP Version: 2.8.16
Operating System: Windows
GIMP Experience: Intermediate Level

List any relevant plug-ins or scripts:
G'MIC Pencil Portrait

List any ERROR messages you received:
none

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I have no problems with most of the GMIC filters in terms of execution except for this particular one Pencil Portrait. The processing counter stopped for a while at 885 seconds, but the twirly indicating it was working kept going, and finally after quite some time the counting continued skipping straight to 1380 seconds. It went on for another 5 minutes like that until finally it stopped at 1658 seconds, producing a rather mediocre black and white (I changed the colour to black) layer. The image itself was 3872 x 2592, not overly large, and contained an aerial shot with half ocean half land. The preview should have warned me as it took quite some time before it came back with a sample, but I decided to go ahead with it anyway. NONE of the other filters in GMIC are this slow, even the advertised "slow" ones. All up this took more than 27 MINUTES to complete. I remember it being the same in 2.8.14, so it is not something that is caused by Gimp (at least I do not think so). Does anyone else have this problem? My Windows version is 7 Home Premium and I am working with an i7 processor and 8Gb or RAM.

yes, that filter is very slow (but I like it)
my test:
- image of 2000 x 1125 (thus around 4 to 5 times smaller than yours)
- default settings
- time: around 5 minutes
Your and my durations almost match