After having done some test-work on the "Sketch" filter, I'm willing to experiment with "Pencil Portrait", a filter written by David implementing a flow suggested in a Jamac4k's tutorial.
It produces always nice results, but again my problems are the performances: this filter also is very slow, it takes around 2 minutes to process an image 729*1096 (thus not big, I would say medium-small image).
I see that during the last period David is not posting, hence probably he is very busy, but as soon as he can I would like to ask him whether there could be a significant improvement on the performances (same as for Sketch).
Spes ultima dea

Just to let people who didn't try yet this excellent filter to have a look, here is one of my first experimentgal outcomes
(original can be found in DeviantArt)

another experiment (combined two layers with different settings)

Superb results! Dinasset. I can't say which one I like the most.

Here my two tries applying Pencil Portrait. I must practise longer, it's clear.

I've realize this vario with pencil portrait that, for my taste, looks better:

∙ Open the pic and duplicate it.
∙ Desaturate (luminosity) → This layer must be selected.
∙ G'mic: output mode → New(s) layer and then:

⇶ Repair → Smooth bilateral → Only set iterations equal to 3
⇶ Pencil portrait (Desaturate layer selected) → Only I increase stroke lenght to 35 set the blend mode as grain merge and opacity ≃ 50%

I love your method Jamac! Thank you so much for sharing it. I did my try on the same picture and in my opinion the result is much better.

excellent, really!