I've tried clicking on run and then refresh. I repeated this process 8-9 times and then the animation in the first example started to lag considerably.
This is dues to a memory/cpu "leak" because when you press reset it doesn't run "clearInterval". This could be done via introspection on the code to "undo" any running intervals/timeouts when the reset button is pressed, but the notion of "reset" is to reset the text, not reset the code, so leaving as is.