Crowds are moving at locked 30 FPS.
But interestingly, they are moving at uncapped framerate during slow motion replays. They look smooth like FiFA 17 on slow mo replays. But on other scenes, they are capped to 30 fps.
Also, if frame rate drops to some like 50 fps, then crowd animations get more jarring and looks like 15~20 fps.
Since they work fine on slow mo replays and used to be uncapped on fifa 17, I guess it's just a bug and can be fixed easily with an update.