30% on the down algorithm. My hat is off to you, sir.

Amazingly enough, it's a different word.

$ time adda_down word.lst 2>/dev/null
dictatorialness by adding c (15 letters)
dissertational by adding t (14 letters)
radiationless by adding s (13 letters)
rationalised by adding i (12 letters)
desalinator by adding r (11 letters)
dealations by adding s (10 letters)
dealation by adding o (9 letters)
dentalia by adding i (8 letters)
lanated by adding n (7 letters)
alated by adding d (6 letters)
alate by adding t (5 letters)
alae by adding e (4 letters)
ala does not need to be derived

real 0m7.080s
user 0m0.000s
sys 0m0.000s

Still not as fast as Perl. Damn.