I have a project that is similar. I'm using the same 4051 multiplexer with 6 1.14 ips displays. I can display text with no problems at a speed that looks like its they are all updating at the same time. The problem I'm having is when I add a sd card some of the displays don't init. If the sd card is removed there is no problems. Im trying to display a jpeg using tjpg_decorder library. The picture will display but very slowly. The other thing I tested is that if i remove the clock signal from some of the displays the image will display fast again. This is a similar problem that was mentioned about too many devices slowing down the clock signal?