A minecart is on a circular track that is almost fully inside the 12x12 spawn chunks, at the outside range.

When the cart passes into the spawn 'lazy' chunks area (7 chunks from spawn chunk) and the player is ALSO at the extreme 'render range' for these spawn lazy chunks containing the cart, massive lag is induced by the recheckGaps routine moving the cart into and out of these lazy chunks.

Thus there is a 2 chunk thick oval, with a radius close to render range, around the affected lazy chunks where the player will experience massive lag.

This also applies to other entities that move into and out of the lazy chunks, such as items in a water stream.