Any advise of what to do?

Had my glow plugs done by a mechanic
but now after i start,for the first 2 to 3 miles
on first and second gear, there's very little power
slog away,need to accelerate very hard and takes 3 to 5 seconds to rich full throttle.also for the same time , when i come to neutrol,the timing is out and takes 3 to 5 seconds to comeback to normal.
VW transporteur 800 special 1996 1896cc.Thanks