I had a very similar problem with my old CDTi corsa 1.3. Whilst it was never really solved completely, at times the car was dangerously sluggish to the point where it stalled at the busiest of junctions. The things it could be is quite an extensive list, although I may have missed something, it should give you some areas to start!
I'd recommend trying the absolute basics and cheapest first.
Whatever reason, all of my diesel cars have run A LOT better if you don't use supermarket fuel. BP Ultimate and Shell V-Power Diesel seems great. OR could can add some diesel treatment into a full tank of fresh diesel and give it a bit of an italian tune-up up your nearest bypass
The MAF (Mass Air-Flow sensor) measures air and if this is faulty then you often get delayed turbo boost. Never go for the cheap no-make ones, get a Bosch and you'll be alright
The EGR valves on modern diesels are prone to stick because of sticky carbon build up + too many short journeys and driving like a granny on a Sunday morning
Give the egr a clean out, if it's too bad to clean it will need to be replaced.
You say you've had the car 3 weeks but did you buy it from a dealer or private? If the former, then you'll have a minimum of a 3 month warranty so let the garage diagnose it without you spending a penny. If you bought it privately, you won't have a warranty but those OBD II bluetooth readers you can get on Amazon/eBay may give you error codes once you plug it in to the car and steer you the right way to solving the problem.