That's because of how events work.
Every tick/frame of execution that the event's conditions are true its actions will be executed.
So in that case, as long as PlayerCo... is overlapping TutorialSign, the text stuff is initialising, seemingly writing only one letter.
As far as typewriting effect goes you also have an example available in the "How do I FAQ" (link in my signature) that works without external plugins, only with the "basics" of C2.