This delay only occurs when switching from the user defined mode to the high/strobe mode. There is a reason for this 0.5 second delay.
With the light on, when in the user defined mode, if you quickly tighten and loosen the head within 0.5 seconds, the light will start ramping so that you can select your user defined light level. If you tighten the head and wait longer than 0.5 sec the light then switches to high mode. Without the 0.5 second time delay, there is no way the light will know what you are about to do after you have tightened the head. Will you be quickly loosening it to start ramping, or will you be leaving it tight to get it into High Mode? It is giving you the time to make that choice. With the light on, once you have changed modes from user defined mode to high (you tighted the head and waited the 0.5 sec delay for it to go bright),you can turn the light off and back on repetively and there is never a delay for the high mode to turn on.
If the light is turned off in user defined mode, you tighten the head to get the light ready to turn on in high mode the next time you push the tail switch, the light will again wait 0.5 sec after turning it on before it swithes from the user defined level to high. Again it doesn't know if you will be quickly loosening to get the light to start ramping in the user defined mode, or leaving it tight to stay in high. Again it is giving you time to make that choice, if you want to do so, although most people will be just leaving it tightened in high.
The bottom line of all this is there is nothing wrong with the light or the user interface. If you want your light to go on instantly in high, let it make the transition to high and leave the head tightened. Repeated switching after that will give you an instant on in high.