"Stop running this script?" -- is this legit?

It's just a popup.

<script language="VBScript">
    Sub myAlert(title, content)
        MsgBox content, 0, title
    End Sub
</script>

<script type="text/javascript">
    myAlert("Web Browser", "Stop running this script?\n\nA script on this page is causing your web browser to run slowly. If it continues to run, your computer might become unresponsive.");
</script>

Either by the code above, which works in Internet Explorer, or an actual slow running script can produce that alert. It is pretty much legit and I believe it rather to be an actual slow running script, because who in his right mind would reproduce that box using VBScript?

It's not scam or malware.

They don't ask such stupid questions, clicking on Yes or No won't suddenly install something on your PC.

At best, it is just a joke that you can safely ignore...

What does the Internet Explorer warning look like?

Execute the following in your address bar and wait for a minute or so: javascript:while(true);