--as to that 'time' factor / though here turned orthogonally re. say, real-time pain:
Turns out that.. euthanasia (for cats, specially) is far from a trauma-free 'procedure'.
Even with the surgeon-friend of neighbor at work, the first try for a vein in leg failed. Further, there appear to be no (injectable) anaesthetics yet developed by the Medico-industrial complex which 'work' very well -- even to dull senses for insertion of the hyarge 2-stage catheter device.
I shall next attempt to discover if Anyone in the Vet Industry, Schools etc. are paying the slightest attention -??- to the matter, in search of YAN patentable nostrum. They sure as hell do not seem inclined to make this search for any altruistic/humane reasons, near as recent experience (including of others) demonstrates.
Nothing is ever 'simple' except for the simplistic-minded majority.
{sigh}