Is 40 Mbps enough for Infinite Flight
It should do just fine. I would suggest playing IF close to the router as much as possible so you get the most out of that 40Mbps, if at all feasible.
Speed isn’t the most important thing with Infinite Flight.
The only thing you may notice if the speed is slow (which 40mbit/s isn’t), is that the loading times may be longer. But once that part is completed, you’ll be fine on speeds as low as 1mbit/s.
Stability is what’s the most important when flying Online.