There's no directly exposed setting since the "reduce quality" turns on a bunch of different flags.
However https://hg.hedgewars.org/hedgewars/file/tip/QTfrontend/gameuiconfig.cpp#l422 which is turned on for the bottom 2 slider values could probably be set by forcing SLQuality in your config to 0x00000800

I haven't tested that.

Rebuilding frontend would be an option too, to allow only turning that on for one of the slider levels, or rebuilding engine to force it on.

Depending on your setup, something like
export vblank_mode=0
hedgewars

Might work too

Update. Looks like the slider flag for vsync doesn't do anything since the SDL2 migration...
https://hg.hedgewars.org/hedgewars/diff/7a7611adf715/hedgewars/uStore.pas#l1.38

SDL2 defaults to vsync.
So that means doing it outside the app.