I got a Teensy 4.1 and soldered an extra 8MiB PSRAM to the underside. The PZ1 6502 now sees 4MiB of memory, which is the maximum possible due to 16KiB banks. Interestingly, I can measure NO slowdown in the emulation even though the PSRAM is a serial memory. I am very pleased!
To test the memory a bit I made some thread-code that changes the colour of one pixel every frame, then yields the rest of its slot-time. The X-position of the pixel is the thread-number, so when I start several threads, I get an informative colour cycling pattern on screen. There is a limit of 63 threads, which feels like more than enough for a 6502-system.