The manifold pressure sensor used with the ECU is very fast. It can respond much faster than is required to track any sudden changes in load on your engine. However, the manifold pressure seen at the sensor input may not change as quickly as what is seen by the inlets to the combustion chamber, due partly to the length of the connecting pipe. This delay can be reduced by keeping the length of vacuum hose between the inlet manifold and the pressure sensor as short as possible. Even with very short vacuum hose lengths there may still be a lag between a transient pressure occurring and the pressure reaching the sensor.
Further, when the throttle is cracked open, the sudden change in pressure forces fuel out of atomisation and onto the manifold walls, so it fails to enter the combustion chamber properly atomised, and the engine hesitates. This can be corrected by adjustment of the Transient Throttle Enrichment tables. To overcome any lean out during sudden throttle movement, a Transient Throttle Enrichment function is used to deliver the extra fuel required.
Note that throttle response can also be affected by poor manifold design. This will occur if the fuel injectors are poorly positioned and the fuel is wetting down the walls of the inlet manifold rather than remaining as a mist.