Interesting comment from Facebook:
The Mister's Next core is a compromise - it's not flawless.
Because its main memory is based on sdram, the memory access is not fast enough to match the Next's sram based memory. It starts to insert wait states at 14 MHz which makes the system a lot slower. You can hear this with a simple basic program that runs at 28 MHz and beeps the same tone with layer 2 on and off. On a real Next there is no change in tone but on a Mister you will hear the tone drop with layer 2 on. For many current programs, this has not been noticeable so far because (1) earlier versions of the Next were limited to 14 MHz and the cpu was slowed to 7 MHz during the active display portion; now the system can always run at 28 MHz and (2) earlier software was written for 14 or even 7 MHz and current software is not pushing the Next hardware yet.
The Mister also has the Arm handle the ps2, joystick and sd card i/o. The arm is too slow to serve the sd card data at full speed so games will only be able to draw data from sd card at reduced rates. This can be noticeable especially if audio or video is played directly from sd card. The playvid dot command, that plays video with digital music, is a kind of double whammy example as it requires running at the full 28MHz rate and it streams video data at top speed directly from the sd card.
It is possible to solve these issues on the Mister with a special zx next core because you can buy a sram add-on module and an sd i/o card that such a core could access directly as on the Next but I think those things are uncommon in the Mister community so almost all users will use the compromise core instead. I believe most other systems the Mister is implementing don't suffer these shortcomings; it's just the base Mister system's hardware that has insufficient bandwidth in some areas to meet the zx next spec.
Beyond that you're talking about the difference between an actual computer and an fpga near-complete clone. Things are always missing on clones because the clone hardware rarely is a superset of the source hardware. Depending on what's important to the user, those missing items may not matter. For example, the zx next core on other systems will be missing many of: rtc, pi zero peripheral, expansion bus, esp wifi, membrane keyboard, ps2 mouse, ps2 keyboard, two sega joysticks, 2 Mbps serial through joysticks / wifi connector, tape i/o, beeper, vga or hdmi, etc.
Anyway the Mister is a great system and I am not trying to talk it down. I'm only pointing out the zx next core is a compromise and not timing accurate on the base Mister especially and that compromise is going to show more as time passes.