I haven't done any benchmarking or anything like that, so I'm mostly guessing when I say that the difference in performance is minimal. However, depending on what you need to do, if you use the DLL's you have access to C++ libraries that are probably much more optimized than anything you can come up with on your own in angelscript.
Then again, most of what you want to do in fonline scripting requires very little processing power anyway.
While most of what you need to do can be done in angelscript, due to the way the SDK works, there are certain things that can only be done with the DLL, so in those cases you have no choice. The opposite is true to some extent as well - you won't get away from at least some basic angelscript coding.