I see this bug still isn't fixed after some years.
I've deployed the Affinity tools throughout a small office of designers, but this issue continues to plague us and prevent any serious switch away from Adobe.
It doesn't matter how complex the document is (eg. a warning label with 4 rectangles, an ellipse, and 4 text elements) - the software becomes unresponsively slow after 10-15 minutes of working. I've tried setting the undo limit as far back as I can tolerate - about 10 steps.
Affinity Designer version is 1.10.5.1342. My workstation has a 12-core Ryzen Threadripper Pro, 32 GB of memory, a Quadro P2000, and PCI-e SSDs in mirrored configuration. It runs Windows 10 21H2, build 19044.1645. Everything else on this system is blazingly fast.