This is a well known bug, but I only saw one report for it, which did not have a good description. Under certain conditions, when you start sprinting, the FOV will repeatedly switch between the normal and sprinting levels. Your speed stays at the normal walking pace, and the particles appear in bursts, while the FOV is in the sprinting position.

I know several different events trigger it, but I have found one way to faithfully recreate it after 1-3 tries. Simply hold shift for a moment, then let go, and double tap W the instant you let go. It occurs almost every time. I've also recreated it a couple times by sprinting over a half slab at various positions. The sneaking method no longer seems to work, and there currently is no known way to consistently reproduce this bug.

This bug is especially annoying for me on parkour maps, since I tend to hold shift while adjusting my position before a jump. Since you move at normal walking speed, I almost always miss the jump.