I managed to find this at least: The penny drops means sudden understanding or realisation of something or other. The expression usually implies a period of

I managed to find this at least: The penny drops means sudden understanding or realisation of something or other. The expression usually implies a period of non-comprehension before the penny drops and derives from early penny slot machines in which the coin or coins took quite some time to drop. That is why the expression is sometimes in the form of ‘the penny has finally dropped’. The OED cites the 1940s as the first appearance of the figurative meaning. Penny slot machines first appeared during the mid-19th century and some alternative sources date the current figurative meaning from at least the 1920s, if not earlier. https://idiomorigins.org/origin/penny-drops https://idiomorigins.org/origin/penny-drops Sounds like your intuition is correct, and the phrase comes in particular from the delay in the penny takes to drop in these machines. I'd imagine if they were faster probably the phrase wouldn't have developed, so an interesting example of how some engineers somewhere who designed the slow mechanism ended up impacting the lexicon.