I'm getting this quite often that I'm debugging a script and am getting the slow script warning. I don't think it's the debugger actually running slow as it seems responsive, so I think it's the actual page triggering the warning.
And in that case it's a bug. :)
*** Bug 34303 has been marked as a duplicate of this bug. ***
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests.
Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.