Playing the game in windowed mode causes lower framerates.

Framerate will dip to 40's while in Windowed mode, however when I switch to full screen it'll reach the game intended framerates.

Specs:

CPU: Intel Core i5-6500
GPU: Sapphire Pulse AMD Radeon RX580 (8GB)
Memory: 16 GB's (1366 Clock)
Disk: SanDisk SSD PLUS 1000GB