The performance is seperated from the respective contract. Long time contracts will help you hold down the salary for that period, for example when you push the strength with a conditioning camp and sign the player before he returns.
Nevertheless, the option of performing bonusses schould be discussed
Does a longer contract promote better performance from a player? The only upside that I can see otherwise is the ability to stretch out a currently available pay rate over a pending increase due to strength going up.