Pokémon is a very text-rich series; none of the main games have voice acting, and each requires a lot of reading to complete. Thankfully, Pokémon Brilliant Diamond and Brilliant Pearl let you adjust the speed at which game text is displayed, allowing you to adjust the pace of the game's dialogue to your preferred reading speed. Here's how to change the game's text speed, along with the different speeds it offers.
First, pause the game by pressing the "X" button. From here, go to the options menu and "text speed" should be the first option you see. You will have three choices here: Slow, Normal and Fast. As you highlight each option, a text box at the bottom of the screen will load the text at the selected pace to give you an idea of how slow or fast the game text will appear.
The speed you select will not only affect the pace of the text loading, but also your ability to skip it. By holding "A" or "B" while talking to an NPC, you can load the dialogue faster than him. If you do this while the text speed is set to slow, the text will still load a little faster than usual, but it's still a snail compared to fast text speed, which lets you fly through conversations.