Not gonna lie, I've been experiencing this issue since 1.56. (At least that's what I think the version was on May 5th or 6th)
There was a major memory related crash that would happen to everyone on the server, on top of the FPS bug.
Got so bad that the server shutdown due to everyone quitting. The crash issues seemed to have been fixed with 1.60 but the FPS bug only got worse.