Why is trust necessary for accepting a memory bullet? It´s just an easy way to deliver information.

I´d rather like to know why all Troupe members are so goddam fast. Both Gon and Killua have been portrayed as extremely gifted individuals in terms of speed. Now, if Nen is used to increase your speed, wouldn´t that have been among the basic lessons when learning about it? Dunno, but imo these villains are too speedy.