Someone will tell me I have this wrong I am sure. But I'll try explain the weird dream I had the other day.
Firstly, time dilation is a thing, the fact that some things (gravity, for example) can mean that in one place time may go at a different rate to somewhere else.
One of the thoughts that then occurred to me is that time dilation must change over a distance, creating a gradient - a rate of change of time over distance. One that would be a great one for xkcd What If? is what sort of time dilation gradient could a person tolerate?
My thought was how the human body could possibly cope with passing through some level of time dilation - what gradient would make sense before blood pressure either way is fatal, etc.
Now, lots of sci-fi has time dilation, for various reasons. Sometimes (like a Star Trek TNG episode) there were these bubbles of different rates of time. Sometimes (like a Star Trek Voyager) there were people walking in one time frame around a room of others apparently "frozen" (but just really slow). In StarGate Atlantis they had a portal that transitioned through a time dilation (but supposedly protected you from the transition effects). The idea of "stopping time" for everyone in an area apart from the protagonist, or just for one person, is very common in sci-fi. The SG-1 I am watching now has time dilation in the SGC over a distance of a few floors of the facility.
The huge issue that occurred to me first was energy - sonic boom type stuff but with heat and light. If an environment has normal time, and is, for example, in daylight, but some person is (near) frozen, the amount of light and heat hitting them, from their point of view, would have enough energy to vaporise them very quickly. Time dilation enough to be noticeable would quickly cause severe suntans from the normal light fittings.
But what really struck me, in this dream, was frequency. Surely if someone is in a time frame that is, say, 40% slower than those around them, they would see things in a very odd way. All red would have turned to blue one way, or blue turned red the other way. If someone is half or twice as fast as their surroundings then they will not be able to see or be seen as light will be shifted out of the visible both ways. Indeed, at any notably larger differences you are either microwaving or x-raying someone just by carrying a torch.
Actually the x-ray one is amusing, as normal light could enter the dilation area, pass through as x-rays, a person, and come out converted back to normal light the other side, so you would see people as skeletons illuminated from behind if they were in a time dilation bubble of some sort. The microwave version is probably less of an amusing sight.
Did I get that right?