The 1200ms was an estimation, but it's definitely closer to 1200ms than it is for 250ms for me. There's definitely a difference in set up here- I'm on a Macbook Pro with an M1 Pro chip.

From a screen recording, I count 53 screen-recorded frames from the apparent start of the animation (which occurs after it's invoked) to desktop widgets becoming transparent (which appears to be the point input is no longer blocked). IINA says the video is 50.582 fps (very strange frame rate?) so that would be ~1050ms.

Of course, that doesn't include any input latency or the display latency, so I also took a video with my phone. I took two trials and I recorded a full 1.08 seconds from key depression to transparent widgets. I did two more with Reduce Motion on and got the exact same time.

I am very curious what your set up is, because I am invested in getting this as close to 8.3 ms as possible.

edit: For comparison, my Linux desktop with a similar experimental set up, this takes about 24ms from key depression to the next desktop becoming visible. The only experimental difference is that I had to switch to the "slow mo" camera to record the difference, and I have a 240hz monitor. The desktop is also considered one of the slower ones (GNOME).

TLDR: It takes 1.08 seconds, on my Macbook, to complete a desktop transition.