Am I the only one to see this as a bug ?
One thing to remember about OpenXCom is that it tries to replicate the experience of the original XCOM/UFO game as much as possible. So repair times on the plane should take as long as they did in the original game. If you want to deviate from that, that's when you'll need to mod the game, or tweak its rules. (both of which are easy to do.)
Personally, I like the way something could be damaged to a degree where it really is going be quicker to build a whole new one than it is to repair it. Things are bent, parts are damaged, other parts may have taken on shock/heat/etc damage. If a craft has been badly shot-up, I can bet parts of the airframe will have to be taken apart, repaired, straightened, patched, etc .. then you put it together again, test it .. oh crap, some other part isn't working as it should .. take that out, discover damage you didn't expect .. troubleshoot, repair, test, repeat .. ..yes, it could take a very long time.