On a diesel engine, to get turbo boost, you need fuel to be burned in the engine. This can be a vicious circle, since in an electronic diesel engine, you need boost to get fuel and fuel to get boost. Most all diesel engines have some sort of turbo boost lag, especially the engines that do not use a variable vane turbo, or the engines that do not use a turbocharger with a waste gate. I am not familiar with the fuel system that is used on the BMW, since they are not imported into the USA, but I am going to assume that is is using a Bosch injection system with electronic controls, much like a Mercedes Benz would use. Those will have a manifold absolute pressure sensor, that must be reporting properly, it also must have no air leaks between the turbocharger and the engine, and it also must not have any exhaust leaks between the engine and the turbocharger. The turbocharger should be inspected for proper shaft/bushing clearance, and closely examine the blades on both the hot wheel and the cold wheel. The cold side blades should all be the same shape, with nice sharp edges and nice sharp corners. If the blades are a dull gray, with a grainy finish, there is a possibility that the turbo has been dusted from being operated with an air filter that was not up to the task of filtering air. Remove the exhaust pipe from the turbo and examine the hot side blades. They should also have nice sharp straight edges with nice sharp corners. If the corners are rounded off, it is an indication of excessive exhaust gas temperature. Any blade wear or distortion will severely effect the performance of the turbocharger, especially the performance on acceleration. If these all test out, then I would recommend that a scan tool be used to analyze the electronic part of the fuel system.