I have used Whatsapp on many different platforms: Symbian, iOS, BBOS, BB 10, and Android. Never have I seen it run with such a lack of optimization and as slow as on Windows Phone. It was a horrible experience 2 years ago on the Lumia 800 and it has improved greatly since. This improvement is mostly due to the faster hardware. Still, it is no where near the speed it should be. The Nokia Lumia 1520 has a Snapdragon 800 on an operating system that is known for its efficiency, yet the second I open whatsapp, heavy battery drain and lag are instantly present. I have read that big updates are on the way but none have showed up yet.
While using the WP 8.1 Preview, Whatsapp was on average 3-5 times slower than when on WP 8. Opening chats took several seconds and sometimes didn’t even open. It was the main reason I went back to WP 8 as it was just unbearable for an app that I use constantly day in and day out.
I am curious though if on sub 1080p devices such as the Lumia 1020 whether or not Whatsapp runs better as it may be optimized for 480p and 720p screens. The icons on the Lumia 1520 within the app are grainy and it is clear that the app is not made for 1080p devices.
I will just put up with it for now and hope for future updates to address the many issues, especially in 8.1 for its release is nearing.