If it's a diesel check all of the vacuum connections on the left side of the engine. It's a common fault that at least one of the hoses is left disconnected and the turbo will not spool up properly as the vacuum reservoir (small black tank on left suspension turret) is unplugged. No vacuum = reduced boost = power reduction.
The reservoir is just visible in the bottom left of this picture (black caged structure) and should be connected all the way through. Changing air filters often disturbs the vacuum lines.
UK Diesel owners have had this on so many occasions!! Of course if it's not a diesel then pass......