LIKE most customers lined 11 deep at Fresco Tortilla shortly after noon last Tuesday, the man in the Armani suit uses the fish face. First, his eyes squint up at the huge 54-item Tex-Mex menu, then his mouth forms an ''o.'' He begins to order an item, then goes back to the ''o'' again. Finally, he orders ''Number 10,'' a grilled chicken taco, to go.

The cashier swivels, gives the order to the kitchen crew behind her, triggering an organized blur of knives, tongs and spoons over chicken, lettuce, tomato and cheese. The man moves to his left and gets his chicken taco. Elapsed time: 35 seconds. ''Oh,'' he says, his surprise now audible.

''This is slow,'' said De Shi Zheng (pronounced duh shuh jeng), who with his wife, Rose, owns the restaurant at 125 West 42d Street and four other Fresco Tortillas. He is not talking about the performance of his six-member crew, all of whom are Chinese immigrants including two of his brothers, but the lunchtime crowd on this winter day.

Despite his success so far, Mr. Zheng worries about the future of his business. A former elementary school teacher from Fujian province whose father was a farmer, Mr. Zheng 33, is in the next phase of an American success story. After he arrived in the United States in 1985, he worked for five years in menial restaurant jobs in Manhattan, saving $5,000 while living in cramped $150-a-month Chinatown apartments with a dozen other men.