Replaced head gasket, loss of horsepower and acceleration

I have recently had a mechanic replace head gasket on my 1997 Chevy 1500 Silverado with a 5.7 liter engine. I have had this vehicle for a long time, however, now it feels like it has less horsepower and acceleration. Also notice a small hesitation when stopped for a while and then accelerate. My main concern is loss of horsepower, takes alot longer to get to 70 mhp or pass another vehicle. Before replace head gasket trouble, the 5.7 would fly. What is happening