Logged into the Flash client and it indeed does feel laggy, but at the same time performance issues are likely more due to Flash itself rather than Tibia.
For some background, Adobe Flash cannot run with multiple cores. It mainly operates on only just one core. If you have hardware acceleration, good for you as it should make the Flash client a little better, but I don't have a GPU to note any performance differences. I'm also not sure about how Tibia handles multi-core systems, but if and if the Tibia client handles it better than Flash, performance issues are going to be evident, but this has more to do with Flash than with Tibia.
If what I outlined above is true (I'm quite certain on the Flash half), then it is natural to note a decrease in performance on the Flash client versus a modern computer running the Tibia client. Of course, it is too early to hate on the Flash client still as performance wise, it could benefit from a means of toggling graphics quality and a decrease in the active scripts running in the background simultaneously.
Performance wise, it still feels clunky and lack smoothness. It is not exceptionally responsive (although I admire the programmers for getting right click control to work decently and effectively; Suck that, aviary!) but it is an alternative to the Tibia client.
I suppose the compromise is to bring the UI changes and updates from the Flash client to the Tibia one. Even though the Flash client is laggier and less responsive in general, I love the UI much more than the Tibia client.