how do i reduce the delay of the taskbar buttons?

I have decided that I want to switch to use combined taskbar buttons. My problem is with the delay of the pop-up menu to show me the different files opene under the same program. for example: file1.txt and file 2.txt with both opened with notepad.

I don't know what these are called so I couldn't search it in google. Perhaps if I attach the screenshot of it here; someone can tell me how to. I know I have to change something in the registry but I do not know which one.

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how do i reduce the delay of the taskbar buttons?