The problem started before the arc. Notice how the angle of the line doesn't point toward the point where it would meet the first pass. Your X-axis slowed way down, but the Y-axis carried on at normal speed. I would suspect the motor drive overheating, the setting of the drive current pots, or the pulley set screws slipping.
EDIT: Looking at the first pass arc, it looks like the X-axis was already having difficulty there. It looks fine for a bit, then goes almost diagonally toward the apex (unless your arc was drawn that way). It seems to get better, when it reverses direction, but after the second arc, it slows down by 1/3rd or so.