I found that with the default setting of gfx_mode=opengl_nearest the game felt VERY sluggish on a modern rather high-end computer, but switching to eg gfx_mode=hq2x it runs great.

Is there a reason why COMI ships with ScummVM configured for the OpenGL option, I'm guessing that option may be rather hit-or-miss (assuming works well in some cases)?
Or am I just doing it wrong if the default setting don't work very well?

I would guess that a lot of people may not even find their way into the ScummVM graphics options (F1 -> Return to Launcher -> Options), or the ini file for that matter.

Maybe just noting this here helps someone else, but do the defaults even make sense in the first place?

Don't forget the F5 also brings up an option panel, from which you can go to launcher options, too. I'm using HQ3x graphics mode and 256-color PC mode for the renderer. With Scummvm games I always experiment with the options...;) Game renders at my native res of 2560x1440p--also use FSAA as well.