Investigate performance of Flights model after fixing issue #2

[alex-lew]

Performance of Flights model suffers without the subproblem block at

f51c948#diff-2a3b7234fcda10bae8f2e3e677e2add7dc29ea841a266f8a13708c4e57ac069bR14

but it is unclear to me why this should be the case: the flight ID is always observed.