turbo lag comes from the compressor not being efficient until it reaches a high speed, say 50,000+ rpm? when the compressor isnt spinning fast enough it doesn't produce much positive pressure or airflow so the engine just is burning whatever air comes in. as the compressor gets faster and faster from exhaust gas/engine load the compressor beings to "compress" the intake and stuff more air in.
when there is little exhaust gas the compressor doesnt spin fast enough, so its referred to as lag. common for the skyline range is nothing before say 2500rpm. past this theres enough exhaust gas to wind the compressor into its effiency zone and usually get around 10psi positive "boost" pressure before 3000rpm.
as engine displacment and type changes, along with turbocharger technologoy, engine tune etc so to does the engine lag. so there are a few things you can do to reduce turbocharger lag