Download and install JPEXS Free Flash Decompiler (FFDec) and use it to open dialoguemenu.swf. 
In the tag list, navigate to scripts/__Packages/<default package>/DialogueMenu. 
Click the Edit ActionScript button at the bottom of the screen.

Recommended method (removing all unnecessary code)
Remove the following code:
static var ALLOW_PROGRESS_DELAY = 750;

In function DialogueMenu:
this.bAllowProgress = false;

In function InitExtensions:
gfx.io.GameDelegate.addCallBack("NotifyVoiceReady",this,"OnVoiceReady");

Remove the whole functions: OnVoiceReady, StartProgressTimer, and SetAllowProgress.

In the first if-statement of function onItemSelect, change
if(this.bAllowProgress && event.keyboardOrMouse != 0)
to
if(event.keyboardOrMouse != 0)
and remove
this.bAllowProgress = false;
at the end of the body of this if-statement.

Change
function SkipText()
{
   if(this.bAllowProgress)
   {
      gfx.io.GameDelegate.call("SkipText",[]);
      this.bAllowProgress = false;
   }
}
to
function SkipText()
{
   gfx.io.GameDelegate.call("SkipText",[]);
}

Use Ctrl + F to confirm that all references to ALLOW_PROGRESS_DELAY, bAllowProgress, OnVoiceReady, StartProgressTimer, and SetAllowProgress have been removed. Click the save-button at the bottom of the screen, then save the file (you may want to save as a separate file to keep the original intact).

Alternative (simpler) methods
There are simpler methods to achieve the same effect, although they leave behind some unnecessary code:

• Setting ALLOW_PROGRESS_DELAY to 0
• Only removing the bAllowProgress conditions from if-statements
• Making it so bAllowProgress is always set to true (this is essentially what Instantly Skip Dialogue NG does)