Only buy a magnetic keyboard for gaming

We'll see. As of the latest testing, mechanicals still show the best single-key latency performance.

I got an ROG Azoth awhile back for that reason.

Would be nice to see how far they can push the technology though!

Given that the time to physically press the key down is a part of the latency, Hall Effect could easily beat out Mechanical in the long run. After all, you can make it almost arbitrarily sensitive.

EDIT: Looks like wired mechanicals at the top end are tied (0.8ms) with the best Halls (0.8ms) and a bit better than the best wireless 2.4Ghz (1.7ms). Mea culpa. We still need more test data on Halls in general though. Removed my comment about Wireless being better - though generally wireless 2.4 GHz beats out all but the top-end wired keyboards.

Lower latency than PS/2?

That's harder to say, there aren't exactly a glut of PS/2 keyboards and mice these days or thorough testing data. And for good reason.

But the best keyboards these days approach around 0.8 ms: https://www.rtings.com/keyboard/reviews/corsair/k100-air