We hit performance issues when writing a dataflow graph editor in SwiftUI [1]. We couldn't express the graph as Views and instead used a Canvas to draw everything. So, we had to do manual layout of everything, and manual event processing. This also precludes putting custom views for user-defined UI inside the modules. The performance problem had to do with evaluating the positions of all the wires between the modules using many GeometryReaders.

[1] https://github.com/audiokit/flow