I don't know if I'm the only one.
During my career, as soon as I use a flashback, I experience a significant drop in FPS.
I run between 140/150 on high, and as soon as I use a flashback, I drop to 40/60 fps with stutters... It's unplayable.
The strange thing is, when I'm replaying my flashback, I experience no lag and I return to my standard FPS.
I can't send a video because it would distort the FPS.