The latency is very low. I just recorded a clip with 120 fps.
Right side is the original machine. Using VGA on a Fujitsu Siemens Scenicview P20-2 which has a delay of 16 ms (depending on how its measured, according to datasheets)
Left Side is a PC, with the Datapath connection, showing it in a window on a IIYAMA ProLite B2280WSD which supposedly has 5 ms delay.
So let's say that the left monitor has a delay advantage of 11ms
And when I am shooting with a gun in Duke Nukem 3D you can see that it looks pretty much the same. If you go into it frame by frame, you can see that the ammo goes down just a tiny bit differently. Not even every frame. Considering one frame on 120 fps is 8ms, it's less than that. Let's assume its 6ms
Maning the right screen (original hardware) takes 16ms to display it.
The left screen takes maybe 16+7ms = 23s to display it. Subtracting the delay of the monitor itself, you have 23s-6ms=17ms of additional delay by the datapath if my math is right. ^^
I think for the vast majority of games, this is very likely completely irrelevant. Maybe if you play shmups or other stuff with extreme reaction times, then maybe it makes a slight difference.
I think youtube only offers up to 60 fps, you can go foward/backward a frame pressing komma , and dot .
Edit: Used the VCS software. Ofc it might be possible that the original software would have even less delay. Didn't test that.