I participated in studies where they administered insulin and glucose intravenously. It is wild how they can reliably drop my blood sugar from high to low within a few minutes. Subcutaneously this takes me hours to do in a stable way.
Not only veins, inhalable insulin like Afreeza is also really quick. Unfortunately it only appears to be available in the US (and maybe Canada?), not Europe/Asia from what I last remember.
Inhalable insulin (which is also very fast acting) iirc only allows a dose of 2 units. If your sugars are 400mg/dl (22ish mmol/l) one or two doses wouldn't put you into a coma if you knew your sensitivity. I'm pretty sure I've read up T1s talking about using it as such.
If you shoot up in your veins or into something more vascular you would have faster onset.