Problem With 2001 Ford Mondeo 2.0l Turbo Diesel 115bhp

The car is very slow.the turbo kicks in very late(about 3500rpm).im thinkin the the air flow meter or something like that.when the turbo does kick in i get alot i mean alot of black smoke from the exhaust but it only happens the first time i go past the 3500rpm and after that its still slow but not much smoke at all.i got a new fuel filter and pump and it is still slow.ford and everything is fine that the diagnosis had no problems found.two different people said the crank shaft or the injectors both are very exp to fix.so im hoping it sould be something else.please help.any questions ill answers best i can.

thank u all