Clearing out a jungle or marsh square only yields one more food. With 8 food needed to grow for a city at size 2, that means 8 turns come as needed for the city to reach size 3 when a settler can get produced after clearing has finished. The AI does stack workers before rails for jobs on unroaded tiles (which I try to avoid most of the time, since it's less efficient overall for workers than one worker per roaded tile). I think I saw that it did take them fewer turns than I expected to clear such a tile. But still, 8 turns for a settler without more production seems like a long wait to me.
When I have cities like that, I've had them put out workers, if they can grow to size 2, until enough of those jungle/marsh squares get cleared for better shield, food, and commerce production. If they can't grow to size 2 in time to produce a worker (like the worker finish time is 10 turns, but the growth time is 20 turns), then I'll have them put out a warrior or more preferably a catapult. Then a worker. And more workers until most to all of those tiles get cleared.
With a jungle/marsh start, I guess I would doubt any claim of "this is too much of a focus on workers, we should make other units else instead!".
