Very fast solving (8)
I believe the answer is:
cracking
I believe this is a double definition.
'very fast' is the first definition.
(I've seen this before)
'solving' is the second definition.
(I've seen this before)
(Other definitions for cracking that I've seen before include "Excellent; fracturing" , "great!" , "Wonderful" , "Solving" , "very fast" .)