I tried a couple of different (cheap) HDMI -> USB capture cards.
I tried going from a Mac (M1 Macbook Pro) to PC and vice versa. Method was to run a simple millisecond counter in Max MSP on the source, capture the video, and show it on the receiver. Take a picture and subtract the value on receiver from the source.
Video resolution was 1920 x 1080 and refresh at 60Hz.
For reference, from PC to Mac using NDI, my average latency was 88ms (roughly 3 frames at 30fps).
From Mac to PC using
https://www.amazon.com/gp/product/B097D5CJ4T/?th=1 average latency was 80ms (a little less than 3 frames at 30 fps)
From PC to Mac using
https://www.amazon.com/gp/product/B08Z3XDYQ7/?th=1 average latency was 120ms (roughly 4 frames at 30 fps)
I'm surprised at the performance of these cheap $20 dongles.