At what length do DisplayPort cables start giving you trouble?

Copper DisplayPort cables can become unreliable as they get longer, potentially limiting resolution or refresh rates and causing flickering or signal dropouts. For most desk setups, the practical advice is to use a passive copper cable no longer than six feet to preserve full display quality.

The article says compromises may begin around 10 feet, with stable 4K performance gone by about 15 feet. Active copper cables can extend the range to roughly 15 to 30 feet, while fibre optic cables can reach up to 100 feet, but cost more: a 15-foot fibre cable can exceed $50, compared with about $10 for a 10-foot passive copper cable. Short cables are especially advisable when daisy-chaining monitors.