Can someone explain to me what this is ?

My issue. X5 35i has a lag when I depress the throttle then pushes the head
back into the seat...is this due to the vehicle weight ? tuning ? or "turbo lag" ?
It may just be my imagination, but , seems more noticeable when I first start
driving. Can this be adjusted ? or, is it driver error ? I would opt for a smoother
start from full stop. I have been driving this vehicle for two months now and the
take offs just don't seem to be improving. Herky-jerky take-offs
aren't my cuppa.

Charles