As I understand it the fl2 is front wheel drive until it detects a wheel slipping then the clutch in the haldex closes giving drive to the rear wheels. As the electric pump in the haldex is running all the time then the only delay is the hydraulic valve closing and multi plate clutch locking up which is quite quick (milli seconds) so sounds like the 4wd is doing its job.
In my experience of the FL2 system, engagement is instantaneous, so the front wheels won't spin.
Like I said, the only time mine spins is when I've pulled the Haldex fuse, after which is spins the front tyres very easily.