The i7-7700K is a powerful processor. With 4 Cores and 8 Threads, and a clock speed of 4.2GHz the 7700K is much more powerful at gaming.
The Ryzen 7 1700, with 8 Cores and 16 Threads @3.0GHz is more oriented towards server use and multi-core workstation use, such as video editing. With a relatively low Clock speed, this processor suffers in games that can only utilize up to 4 cores, such as fortnite.
On a side note, the amount of FPS you are seeing is great. There is really no cause for concern here.