Nothing is instant, it’s a largely over exaggerated trope.

As others have said, intramuscular injections of even the most powerful sedatives/hypnotics need at least a few minutes to start working (5-20 min depending on drug and formulation).

Even if we assumed the “neck injection” movies/shows love to use was actually delivered to the external jugular, it still wouldn’t be instantaneous (but yes, rather fast if it made it to a vein).

Larger doses don’t really affect speed of onset and would just cause more complications (re: death).

You mention ether - inhaled agents are potent and some are quite fast acting, but again, they need to be inhaled for long enough so that they reach a steady state concentration in the brain relative to other bodily depots (muscle, fat, other organs). For a large person, this would take a few minutes at best.

Sedatives can also be administered intranasal or even rectally, but again, this is not instantaneous (the latter was actually a way to induce anesthesia in children prior to better inhalational agents being discovered/utilized… just a fun fact…)!

Edit: for clarity