OK, for the flat turn merge people, listen up. All things being equal (plane, speed, pilot skill, etc.) the fastest merge turn is... vertical!
The second slowest is the flat turn.
Since there are only 3 directions available, it's also the best after the vertical. The very slowest is the downward turn.
The primary reason for this is simply gravity. Without going into all the additional setup and related info in detail, essentially going vertical is faster because you use gravity to reduce your airspeed to the proper Corner Velocity for your aircraft and, using throttle control properly pull your nose through the apex of the turn quicker, regaining airspeed as needed as you come back down. Your actual turn radius is very short, and you turn quicker getting your nose around the 180 dgrees, and with E to regain your airspeed quickly.
In a flat turn, gravity remains as a constant drag stealing your E, and eventually reduce your airspeed. And what E you lose in the turn you do not immediately regain.
Now some people might think that a downward turn (usually in the form of a Split S) would be the fastest because initially gravity would be helping to increase your airspeed. However, too much speed is bad as it causes a wider turn radius even if you shut throttle first, plus you are now lower than when you went into the merge.
This is why in most dueling merges you see 90% of the pilots going vertical. Now, that is not to say that there might not be a specific tactical situation where a different turn was better, like the other guy makes a real dumb move that only a flat turn will put you into position quicker, but by and large, the vertical turn is the fastest and best. Master it!
One of the things a pilot should be aware of is that, though it has its uses, airspeed (meaning high airspeed for your aircraft) is your enemy. It's only good for getting to or from a fight quickly. Having the right airspeed is more important.
In conjunction first, another very important concept to learn is Cornering Velocity which is simply the correct or optimum speed for a particular aircraft to turn (thinking initially a flat 180) allowing it to do it in the quickest time which is a result of the right turn radius at the right speed. You should learn the Cornering Velocity for any aircraft you dogfight in.
