So the question we're trying to answer here is how fast the Flash's top speed is right? If that's the case then there's actually a lot of ways to answer this question.
In Flash: The Human Race, the Flash was able to beat instantaneous teleportation by absorbing the kinetic energy of everyone on Earth. By going this fast, the Flash ran faster than a planck instant therefore breaking what we call "speed" and "time". A planck instant (called planck time in real life) is the smallest unit by which we measure time and speed. So when the Flash ran this fast, he was everywhere and nowhere at the same time.
So to beat instantaneous teleportation, the Flash had to go faster than a planck instant which leads me to say that instantaneous teleportation has to be a little slower than the speed of light.
Before I get into more, I want to say that teleportation does not equal moving at high speeds, it's the transfer of information from point A to point B. This transfer, although seen as instantaneous, will still take time.
Moving back to what I was saying before, teleportation is not faster than light. Einstein says that no information can travel faster than the speed of light. It's just not possible.
But we go back to the Flash, he runs faster than a planck instant which is way faster than light. This is possible for two reasons:
1. It's a comic
2. He is not actually transferring info. He's just running faster than light by tapping into the Speed Force and absorbing the kinetic energy of all the people on Earth
So there's my opinion on the Flash's top speed. However, we can also argue that the Flash doesn't really have a speed cap because there will always be the Speed Force. It literally says that since Barry Allen created the Speed Force and generates it with every step he takes, he can "reach any speed he imagines". If he wants to go faster than he just needs to generate more speed (sounds dumb and obvious, I know). The Flash's speed cap is therefore, unknown. We do know that he doesn't like to run faster than Mach 10 (7672.69 m/h according to google conversions) in Earth's environment because the results will be catastrophic.
Just to let ya'all know, it was Wally West that ran faster than a planck instant not Barry Allen.
Also here's a list of some conversions:
Mach 2- 1534.54 m/h
Mach 10- 7672.69 m/h
SoS- 767.269 m/h (343 metres per second)
SoL- 671 million m/h (exact value is 299,792,458 metres per second)
Planck Instant- Undetermined. This is unable to be measured by the current technology that we have today. "As of May 2010, the smallest time interval that was directly measured was on the order of 12 attoseconds (12 × 10−18 seconds), about 10 24<---this is suppose to be an exponent) times larger than the Planck time."- Wikipedia