I've had mine for a little while now and I can honestly say that most third party apps struggle. If you haven't used an app for a while it will take at least 5 seconds of spinning before you are shown anything. From there, depending on the app, you may have to wait more before you can actually do anything and you'll have to wait more again if you actually do do anything. It's almost always a few seconds wait for anything you do apart from scrolling a list of stuff which is usually nice and smooth.
For me, in many cases, it's about bad enough that I may as well get my phone out of my pocket and use that instead.
Apps that have buttons to press (which is most of them) are also often clunky. You can't quickly tap a sequence. Press, wait, press, wait... I have a calculator app and I'd probably be quicker working out the sums on paper if I needed to use it.
Don't get me wrong, I love my watch and the native Apple apps are nothing like this. The photos app for instance, I have 200+ pics on there and it loads in less than 2 seconds and is super smooth. The music app pops up so quick I can't time it, same with email and maps though it takes a little while to get my location.
I'm just really hoping that when the native app SDK comes out it will come to V1 of the watch.