In old plays and stories, such as Romeo and Juliet, poisons are depicted as being fairly fast acting.
Would they really have had access to such poison, or was it simply creative license? What would a realistic depiction of a poison of that era be?
yes, herbalists and healers would have access to and knowledge of fast-acting poisons.
there are plenty of plants like nightshade that can rapidly kill a human, and those familiar with plant life would be well aware of that.
hemlock isn’t as fast acting, maybe a few hours before death if you ate it, but that’s how Socrates was executed 2400 years ago, and according to sources it took about half an hour for him to die, so knowledge of fast-acting poisons is very old.
https://en.wikipedia.org/wiki/Taxine_alkaloids
An extremely common tree that people grow as hedges and cut into fun shapes.
In animals the only sign is often sudden death.
As you correctly surmised, most poisons wouldn’t act nearly as fast, although there are exceptions. In reality, people would just get very sick for a very long time and perhaps not recover. It would’ve been difficult to even correctly identify poison over illness or food-borne pathogens. I’m willing to bet a lot of people who died of natural causes had their deaths attributed to poison because of their position and lack of medical knowledge as well.
Then again, some poisons and their effects were well-known to contemporary physicians or their forebears, so depending on the precise effects they could possibly deduce what happened in some cases.