Turbocharged engines provide a non-linear power delivery – they can feel slothful at low engine speeds, then eager at high speeds. There are two different causes, both of which are often referred to under the single term 'turbo lag'.
Turbochargers will always be capable of providing more boost at higher engine speeds, because the exhaust flow that powers the turbocharger will be greater when the engine is spinning more quickly. At low engine speeds there is little exhaust gas flow, so the turbo spins slowly and provides little (if any) boost. When the engine spins at high speed, the turbocharger receives plenty of exhaust gas and it, too, spins at high speed – compressing more air into the cylinders.
Modern turbocharger installations use a variety of techniques to allow the turbos to work at low engine speeds – strategies such as limiting turbo size, using variable-geometry turbos and using compound turbo installations (one turbo feeding another).
The second problem, true 'turbo lag', occurs when the engine is run at low load (ie with the throttle closed) and then the throttled is snapped open. With the throttle shut there is little exhaust flow, and the turbo spins only slowly. In the split second after the throttle is opened exhaust flow increases, but the turbocharger cannot instantly respond because it has some interia – it takes time for the turbo to spin up to working speed and build up boost pressure. The driver feels this slow response – turbo lag.
Small, low-inertia turbos combat turbo lag, providing turbo engines with better drivability.