Simply speaking, turbo lag is when the turbo has not spooled enough to produce compressed air. It is the delay in the response between the process of accelerating and the response of the engine or turbo.
When the turbo is not at maximum pressure or load, it will release pressure to reduce the amount of damage it could cause. When the accelerator is applied, the turbo needs to rebuild the pressure with gases from the exhaust to produce full boost.
Engine responsiveness is an important factor when driving. Some lag only lasts for a millisecond whilst others could be for a couple of seconds. The lag is noticed by how slowly the car is going without gaining acceleration as quickly as it is supposed to.
Sometimes the car has a turbocharger installed that is too large. A smaller device will require less pressure and will, therefore, produce more boost.
Friction could also influence the performance of the turbo. When less friction is present, less effort will be required to spool the turbo which means that lag may be reduced.
Installing the turbochargers closer to the exhaust outlets will also ensure that lag will be reduced. By placing the device closer to the exhaust outlets, the distance in which the exhaust gases need to travel has been shortened. This means that pressure has not been lost in the process of traveling from point A to point B.
Another factor that causes turbo-lag is the weight of the moving parts within the charger. The weight of these parts will need a greater force which could influence efficiency of the device. If efficiency is what you are looking for then it is necessary to have a turbo rotor made of lightweight alloys. Compact models with smaller rotor diameters of the turbocharger will also require a smaller centrifugal force that will allow the car to accelerate faster without producing any lag.