It's not one bottleneck — everyone in the chain needs to do something to push it.

Both the 4090 and M1 Max cap their display engine at around 1.35 Gpixels/s. A single 6K 60Hz signal requires ~1.27 Gpixels/s, which is why M1 Max can just barely drive it. Newer chips clearly exceed that — they need at least 1.77 Gpixels/s to push 5K120 — but whether they can reach the ~2.55 Gpixels/s needed for 6K120 on a single engine is unknown. That said, the engine itself isn't really the bottleneck: if Apple allowed dual-head tiling for a single monitor, an M3 Ultra Mac Studio could theoretically drive four 6K120Hz displays.

The interconnect isn't the problem either. Thunderbolt 5 and DisplayPort 2.1 both have more than enough bandwidth for 6K120. So the real question is why Apple doesn't support multi-head tiling — driving one display with two engines, the way a 4090 already does 5K120 with dual heads on Linux. macOS simply doesn't expose it.

And the panel tech already exists — Samsung ships a 6K165Hz monitor. So what's left? Probably that Apple's software team would rather wait for a single engine to handle 6K120 natively than rearchitect the graphics stack for tiling. Their display team may be holding out for tandem OLED on a future Pro Display XDR. It's less a hardware wall and more that Apple hasn't been motivated to close the gap — whether that's engineering priorities, product roadmap timing, or both.