Three airports serve Paris and people keep treating them as one.

That is the mistake that costs money here, not security queues. Charles de Gaulle sits 25 km northeast, Orly 18 km south, and Beauvais is 85 km north of the city in a way that makes calling it a Paris airport almost rude. The coach from Porte Maillot to Beauvais takes an hour and a quarter and costs about €17 each way. So a fare €25 below the Orly equivalent is really €9 below it, and you have spent three hours of your day getting even.

Check which airport your fare uses before you compare anything. Kupi.com — flights from Paris lists them together, which is exactly what you want — as long as you look at the airport code and not just the price.

The transfer, honestly

RER B to CDG: about 35 minutes, roughly €11, and it runs from around 04:50. Métro line 14 now reaches Orly, which has quietly made Orly the easier of the two if you are staying on the Left Bank. Both beat a taxi, which will be €55–65 to CDG and slower at the times you care about.

If your flight leaves before 06:00 the train question answers itself. There is no train. Either pay for the taxi or accept arriving absurdly early.

Terminals, and the one that gets people

CDG's Terminal 2 is seven halls lettered A to G. Some are a ten-minute walk apart. Two need a shuttle.

The trap is the satellites off 2E. A boarding pass saying "2E" does not tell you whether your gate is K, L or M, and the walk out to K4 is a genuine fifteen minutes, longer when the moving walkway is out, which it often is. If your gate starts with K, L or M, take that quarter of an hour off whatever margin you thought you had.

Terminal 1 is the circular one with satellites underneath. Terminal 3 handles charters and looks like what it is, a shed.

Orly is four halls under one roof now and the walk from kerb to gate rarely tops ten minutes. That difference is worth something concrete: a 07:00 departure from Orly needs a 05:15 start, the same flight from CDG needs 04:45.

How early, actually

The official line is two hours Schengen, three for the rest. Follow it literally and you will spend a lot of your life near a Paul.

• Cabin bag only, checked in online, Schengen: 75 minutes at Orly, 90 at CDG.
• Bag to drop: add half an hour, and more if the airline uses self-service kiosks. They fail. You queue twice.
• Long-haul or non-Schengen at CDG: the full three hours, and not because of security — passport control before the satellites backs up hard.
• Between 17:00 and 19:30 at CDG: add thirty minutes to anything above. That is the transatlantic bank pushing off.
• Flying before 06:00: margin is irrelevant. The constraint is the first RER.

What goes wrong in practice

Not security. It is duller than that.

A cabin bag over the limit at the gate, which is €50 and a bad mood. A passport with under three months' validity for a non-EU destination, which ends the trip at the desk. And the walk to the gate, which is why people run.

One more, specific to CDG: the RER stops at two stations there, Terminal 1 and Terminal 2. Getting off at the wrong one costs ten minutes and a shuttle ride. The signs are fine. At 05:30 nobody reads signs.

What settles it

The airport that matches where you slept, not the one with the cheapest fare. Once you price the transfer at both ends and the hour you lose to Beauvais, the "cheap" option usually is not one — and a mid-morning departure from the right airport costs about €20 more than a dawn flight from the wrong one while removing every failure mode above.

Have a look at how the mid-morning fares sit against the 06:00 ones on your dates: Paris flight deals across a month will show it in about thirty seconds. Often the extra sleep is free, which is the only travel hack on this page I would defend without qualification.