Ah, you're cranking the Cost Index numbers out?
A Cost Index of zero will save you fuel per mile flown but crew and maintenance costs will go up per mile. On a short leg it's hard to gain anything by going fast (you could cruise supersonic DTW-CLE and I doubt it it would make a minute's difference) but on a 14-15 hour leg balancing the fuel burn v. when the next "C" check comes due can matter. You might be able to squeeze some more legs in and therefore make more revenue.
Define aircraft performance. You'll go slower. That's reduced performance. Are you hurting anything or is something going to break if you put in "0"? No.