How fast can a signal go from Kinshasa to Washington, D.C.?

No faster than 35.11 ms one way and 70.22 ms there and back. That is light in a vacuum along the 10,526 km great circle between the two cities. Optical fibre slows light by its group index, 1.4682, so through glass the same round trip takes at least 103.10 ms.

The great circle from Kinshasa (4.38° S, 15.33° E) to Washington, D.C. (38.90° N, 77.10° W). Equal Earth projection; land from Natural Earth.

Floors from Kinshasa to Washington, D.C.
Medium	One way	Round trip
Light in a vacuum	35.11 ms	70.22 ms
Light in glass, group index 1.4682	51.55 ms	103.10 ms
Great-circle distance, WGS84	10,526 km

What does a real line add?

Cables leave the great circle, and every router and queue on the way adds time, so a measured round trip from Kinshasa to Washington, D.C. lands above 103.10 ms whenever the whole path is glass. A line that only just reaches 103.10 ms runs at 68.1 % of light, the most fibre allows.

Time Your Own LineYour connection, graded against light, from your browser

How are these floors computed?

The distance is Lambert's formula on the WGS84 ellipsoid between the population-weighted centres UN DESA gives for the two cities in World Urbanization Prospects 2025, checked against GeographicLib to within 100 m on ten pairs. Light covers 299,792.458 km each second. The glass figure uses Corning's group index for SMF-28e+ fibre at 1550 nm, 1.4682.