it sounds like the carb is not metering fuel according to the engine's needs..
When you open the throttle wide, a lot of air suddenly streams in. This air should instantly draw lots more fuel up and into the airstream. If fuel is restricted there is a power lag while the engine only gradually speeds up because air:fuel mix is off and power is low.
So, assuming the bike is 100% stock, the problem seems to be that incoming air is not being matched with an adequate amount of incoming fuel. My guess is the main jet is clogged or the float level is low. So, it might be best to just clean and tune up the carburetor. Also look for air or fuel leaks in and around the intake manifold. And make sure fuel flow is good from the tank to the engine.
The bike was not built that way and your problem is an abnormality. Unless this bike was modified, returning the carb/fuel system to as-new condition should cure it.