Almost certainly due to bandwidth limitations on older versions of Thunderbolt. Full bit depth HDR 5k @ 120hz requires some absurd data thoughput.

I don’t think so. My M3 Pro is on the list as supporting 120 hz but it only has Thunderbolt 4.

Also the base M4 doesn’t habe Thunderbolt 5 and it support 120 hz.

> My M3 Pro is on the list as supporting 120 hz

Can you point me to said list? All I could find was:

> Mac models with M1, M1 Pro, M1 Max, M1 Ultra, M2, and M3 support Studio Display XDR at up to 60Hz. All other Studio Display XDR features are supported.

And The Verge reports:

> There’s also support for adaptive sync that can adjust between 47Hz and 120Hz (if it’s connected to an M4 Mac or later, or the M5 iPad Pro)

I got an M3 Max and was strongly considering upgrading my old monitor, but if I can't do 120hz, I'll just wait until I upgrade my laptop as well.

I’ll give you an anecdote: my work laptop is an M3 Pro MBP, and my Dell U4025QW works just fine with it over Thunderbolt at 120Hz VRR

That monitor has a noticeable lower pixel count.

Dell U4025QW: 5120 x 2160 = 11,059,200 vs Apple Studio Display XDR: 5120 x 2880 = 14,745,600

So your display has 25% less pixels.

It’s quite possible this is running with a reduced color space (chroma subsampling). Degradation happens automatically based on available throughput and most people don’t notice.

For desktop use? Chroma subsampling is obvious. DSC compression, on the other hand, is not. DisplayPort and HDMI support both.

It’s obvious if you use a test pattern and/or know what to look for: https://testufo.com/chroma

I had no idea what it was for the longest time. As it turns out, macOS frequently enables it even when it’s unnecessary, and without any way to override.

> Can you point me to said list?

There’s no list per-se. The MacBook Pro (2021 and later) is listed as supported. The M3 Pro and M3 Max are not listed as only supporting 60Hz while the M3 and M1 Pro are.

They did say M3, not M3 Pro. You're probably okay.

(Notice how they listed the M1 chips individually.)

I don't really see your point. The chips mentioned do not have enough bandwidth on display outputs to support the monitor at 6K@120Hz. If anything, I find it surprising that Apple supports running the display in 60Hz mode instead of telling people to go pound sand and buy new Macs.