We are developing iOS and Android using Titanium.
IOS
IOS apps are very fast. You will not notice that Titanium is used if you do not need concurrency. If you want to add rows to the table view using Pull-To-Refresh or when scrolling it will be slow because you cannot control when to use UI-Thread and when not. It is processed by titanium. Nevertheless, our application uses these features, and we - and our (business) customers - are satisfied with this.
Android
Android can be a lot slower. It depends on your device and what you are going to implement. For example, again, a table view with many different (in structure) rows can be very slow, while a table view with thousands of similar rows loads quickly and can be scrolled quickly (see the class attribute for table table rows, for caching). On new devices (originally shipped with Android 4.x), performance as well as native development (except in some special cases, such as iOS). Older devices are much slower. On Android, the table view is very slowly populated using HTTPRequest data. We have not yet figured out why this is so slow, but - depending on the device and network - loading 10 messages and filling out a table view sometimes takes up to 5 seconds, even under the best network conditions. But this is the only real performance issue that we have noticed so far.
For both: Finally, it depends on the use case that you want to implement. To find out what is possible, you should install KitchenSink apps (available on the iTunes and Play Store for both systems) and give it a try. You will also see their performance. But be careful - of course, these demo apps are optimized to convince you to use titanium. But they are open source, so you can watch and see how they did it.