1991 Kia Pride car is slow when warm

I have this problem before with my old engine so i decided to change my original engine to mazda z5 dohc engine to see if my problem will be solved still the problem is still there every time i use the car it runs smoothly at first but after around 15 mins the car still run a little bit slow than normal what could be cause of this by the way the mechanic used my old tranny, my starter and my alternator