I didn’t do a full matrix of what’s incompatible, but I’ve confirmed that using Wifi + Flash filesystem + SmartMatrix causes occasional hard hangs on the CPU.
There is probably some race condition deadlock between interrupts, paging Flash IO through IRAM and paging PSRAM through IRAM. I don’t know ESP32 enough to dig deeper, but I’'m reasonably sure that turning off PSRAM in Smartmatrix, fixed my problem.
Either way, PSRAM is indeed a bit slower (not that I noticed), and there are likely reasons not to enable it by default on ESP32, even when available.
I just sent
to @Louis to make PSRAM an option just like teensyv4 (#define SMARTMATRIX_USE_PSRAM before including SmartMatrix)