There are two things you need to be aware of:
A saccade is a fast movement of an eye, head or other part of the body or of a device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction. Saccades are the fastest movements produced by the human body.
The peak angular speed of the eye during a saccade reaches up to 900°/s in humans! Saccades to an unexpected stimulus normally take about 200 milliseconds (ms) to initiate, and then last from about 20–200 ms, depending on their amplitude. The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to about 60°, the velocity of a saccade linearly depends on the amplitude (the so-called saccadic main sequence).
In saccades larger than 60°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye. For instance, a 10° amplitude is associated with a velocity of 300°/s, and 30° is associated with 500°/s. (Source_1, Source_2)
So, 0.2 seconds to initiate, 0.02 seconds best-case. Anything that changes position in less time than 0.22 seconds can't be tracked by the human eye.
I'm not even going to bother quoting anyone or listing citations. So many people are comparing apples to oranges on this issue that it's hard to boil down useful citations.
Here's the argument: if your LCD screen is 2,000 pixels wide and a single pixel is moved from one side to the other at the best light-and-decay rate the screen can produce, can the human eye see it?
Yup, it can, and that can correspond to thousands of "frames per second."
What most people ignore is that it's just one dot, you're focused on it, and the actual "velocity" of transit is very slow. For example: if black-to-color-to-black is 1ms, you're only talking about 2,000ms or 2 seconds. If your screen is 20" wide, that's only 0.06 mph, which is walking speed. If it's 0.01ms, then it's 6 mph or very high-quality sprint-running speed. Do you see my point?
So, when we talk "frame rate" of the human eye, we need to talk about how fast the entire picture can change (or something major within the picture) and the eye still see it? That brings us back to Saccade movement, because it's really a question of how fast the brain-eye combination can (a) notice that something needs to be tracked, (b) lock onto that object, and (c) move the eye to maintain focus.
Remember that 0.02 seconds? That's the fastest (give or take, no two people are the same) the eye/head combination can track something. The 0.2 seconds is the time needed to notice and lock on. Your superheros need to be out of the frame in that time (otherwise the eye's peripheral vision is still tracking them).
This depends on how far away they are, which you didn't mention. The closer they are, the slower they can move and be out of frame before the eye can catch up. So, for illustration purposes, let's pick numbers.
Let's use the above reference's 60° number for the maximum Saccade speed. If they're 100 yards away (and simplifying this to a triangle calculation rather than an arc calculation, which would maximize the distance, but it'll be good enough), then they need to travel 100 yards in anything less than 0.22 seconds.
That's anything more than 1,363.64 feet/s2
And if you're firing a .22 caliber rimfire rifle, that really is faster than a speeding bullet.
And, like any good bullet, you didn't see him move. He was just gone.1
Edit: Let me add a bit of context. Once, while living in Texas, I had the privilege of watching the Shuttle descend toward landing. It was (literally) like watching God draw a line of fire in the sky. It was moving a whole lot faster then 1363fps - but the size of the corridor of burning atmosphere and the distances involved were so great that I could comfortably enjoy a once-in-a-lifetime spectacle. Which is a lovely way of saying the necessary speed is completely situation-dependent.
1 To be fair, the biggest reason you can't see a moving bullet is its size. Humans can see "tracer bullets" because the (I believe it is) burning phosphorous creates a much larger visible target (light glare). In a nutshell, the bullet is easier to see. Your superhero is huge, which means my analysis might be wrong because I'm not taking into account the effect of the size of the target object on visibility.