Apple has not released any information on a new iPhone. We don't even know it will be called the iPhone 8. It might be the iPhone 7s. And no one has it, so there's no experience with it to say how fast it is or will be. I suspect it will be fast enough.
As @Ledsteplin said, we don't even have a confirmed name yet so things are very early at this stage
Im sure we can say with certainty that the next iPhone (7s or 8) will be faster than the current version. But how much faster is unknown right now
Here is a graph of the previous iPhones are their performance between each other.
Apple is very secretive about releasing any information about their future products. Based on what they have done in previous product launches, it will probably be a 15-20% speed and better screen and graphics. Now it's wait and see.