Explain to me how it would slow down the server. To slow down the server you would need this effect to increase the number of aircraft in the air throughout the arena. If this was implemented, there would in fact be zero additional aircraft up at any time, since players wouldn't be able to take off again until their plane is destroyed.
There are exceptional cases to consider. A well trimmed plane in level flight will just continue on until the fuel is exhausted, so a reasonable time limit before respawn after pilot death would be fine, in case the aircraft takes too long to crash. 30 seconds or something, after which the plane dissappears/explodes and you go back to tower.