I think every cause of lag has been touched on here. Good discussion. To summerize:
Lag is subjective and can be caused by many things. Turbo lag is intrinsic to all turbos. Turbo performance is always a compromise. Smaller ex. housing better spooling but higher drive pressure and EGTs on top end. Bigger is better for top end performance. That's why twins (compound/sequential) turbos came about. Best of both world. If you really want to see lag, watch a 1000hp sled puller with a large single. Some of those don't light up until well after 2000rpm. (but they run down the track at over 4K.) Maybe the CRDs turbo is a bit big (looks tiny to me), but as mentioned, it would be a headache to change it out with all the electronic controls. It's odd that this turbo would be big from the factory sense most of the time stock turbos are too small (emissions again). Turbo lag is problematic not only for performance reasons, but also emmisions. That's the reason for the VGT in all the new diesel. Ford opted for twins instead. Not sure how that's gonna work. Very complicated for a stock work truck plus they have them backwards (small turbo first).
Engine tuning: The injection timing has a lot to do with how quick a diesel gets going (and lights the turbo). For many years now, due to the EPA, diesels have had retarded timing for emmisions. This makes it take longer to get the engine rolling. Timing also changes related to throttle position. At low rpm, heavy throttle, more retarded timing. Ever notice how if you stomp it, it takes longer to get going in the first second that if you just ease into it. If you want to improve this on a modern diesel, get a module that advances timing. (Plus you'll get better mileage and lower smoke.)
In addition to timing, is torque managment. The manufactorers tunes the fuel delivery so as to be gentle to all. The ecm fuels slowly off the line and defuels at shifts to save the tranny. That's so grandma can't accidently smoke the tires and loss control on a wet road. There are programs out now for the truck market that modify or elimate the TM altogether. This allows more fueling down low and great throttle response, faster spool and big fun factor. Trade off is more low end/low boost smoke and very hard on tranny.
The outside conditions have effects also. Not just the temp of the fuel, intake air temp, etc. itself, but the ecm changes parameters (such as timing) based on air temp, engine temp, etc.
Transmissons also has an effect. In an auto, a looser torque convertor will get you off the line quicker but at the expense of everything beyond a few 100 ft off the line. A tighter TC will take longer to get everything going but will do very well once up to speed. My 01 Dodge is somewhat over geared for the tire size and has a tight TC. Is a little doggy off the line. In addition, the gear selection makes a difference also. Again with the Dodge (but I think this is common in all Chryslers), the tranny will not shift 3-2 based on speed. (It will based on throttle position, but not speed). It will shift 3-1 at 10 mph. So, when I turn a corner and slow to 15 mph or so, it's in 3rd gear. With the gearing and TC it is a total dog. So I have to give it enough throttle to make it shift 3-2. Anyway, while the tranny is shifting/figuring out what gear to be in, there is some lag. No matter what tranny you have, if you punch it in mid shift, there will lag.
I think that is everything I can think for now. As you can see, there are many things that can cause the subjective lag. So if you want to get rid of lag: advance timing, small or compound turbo, low gearing, cool intake air, warm engine and change the torque mangement.
