What WiFi setup are you using? AP or Client? I’ve been able to use leader follower setups of 6 PBs very reliably. Using multiple PBs have addressed my initial issues of low fps on larger setups. I’m curious if there is any network overhead issues that could be causing slowdowns.
I concur that using delta instead of time() is an issue especially when you have a PB that has even a slightly different FPS due to LED type or count. Those differences in delta add up and while the patterns may look great for a while the errors add up after a while. If your use of the pattern is short you can still get away with using delta.
As for mapping over multiple PBs I found this forum entry to be incredibly useful: Splitting pixel strips and displaying different patterns - #15 by Vortexfractal