Puzzle ZFEX 

Workings

Hint
The answer is in the words given.

Answer
Slow.

Reasoning
Take the third letter of the first word
   + the second letter of the second word
      + plus the first letter of the first word
         + plus the fourth letter of the second word.

Double-Checking
3-1- [1234] -2-4
s-f- [fast] -a-t
o-s- [slow] -l-w