Post is from 2018 and this issue still is somehow not fixed yet, but regardless here's your fix: https://www.reddit.com/r/fo76/comments/a0lc1o/borderless_window_mode_fps_problems/
Raising this with the team! Thanks for bringing this up.
This thread was added on October 04, 2022, with posts from Ladydevann.
Original Post
Raising this with the team! Thanks for bringing this up.