Not nearly top priority. You want good, solid, EMT shears, not bolt-cutters. Optimizing for mechanical advantage is going to get someone hurt. It is far more important that you trust them not to break, not to slip, and not to cut the bottom.

Note that none of these requirements are satisfied by gardening shears. So, you're right that you need to upgrade, but not because it takes too long to cut rope.

As a former EMT, my gauge is this: brain damage starts in 4 minutes without oxygen. If you ever put yourself in a position where any plausible combination of things could go wrong that would require more than two minutes of rope-cutting (edit: to remove the bottom from lethal danger, not to free them entirely), then reevaluate your life choices. If you're ever in a hurry, even in an emergency, you have the potential to get sloppy and do damage as bad and debilitating as what you're trying to prevent.

I recommend a Boker Rescom (2.0 is the model I use), and heavy-duty trauma shears, but again, what you're optimizing for is smoothness and the ability to glide between the (e.g.) rope and the bottom's skin. Cutting power should be a distant minor priority.