Not sure if it's a bug or just a balancing issue. But when I came back to the game this morning, I had the Royal Navy Recruitment event ten times (together with all the other events that triggered over night) - and I discovered that the debuff doesn't stack. So I got 50 k gold (5k per recruitment), while only using 20% speed, basically doubling my lifetime earnings at the cost of one hour more on a trans-oceanic delivery.
I don't know if the 5k is a fixed price, or if it's based on stats/fleet/current job. But I feel like it is (in my current situation) such a high reward, that it shouldn't happen more than once a day. Or the debuffs should stack - though I assume it'll feel like getting forced into battle with the royal navy at some point, which could be bad.