This is what I'm asking, how big can we make the panel before it starts to show visible lag. How far can one data line reach before a section starts to lag behind and make the video deform?
I did that calculation in my answer at the end but as you didn't specify which type of pixels you have/plan to use I simply used the faster version. With that version you might see the delay at about 400 Pixels. But as you didn't define what a visual lag is I simply used a safe value of 50Hz. I don't know if an ESP8266 is able to handle about 5 pins simultaneously updating in the fixed frequency the pixels need. I doubt that there are ready made libraries that can handle that without flickering.