Dane DeValcourt

Light travels through a vacuum at a speed of 186,000 miles per second and electrons travel through copper or fiber at approximately 125,000 miles per second.  A fiber network stretching halfway around the world (13,000 miles) induces a one-way delay of about 70 ms.  Although this delay is almost imperceptible to the human ear, propagation delays in conjunction with handling delays can cause noticeable speech degradation.

Source:  Voice over IP Fundamentals (Second Edition) – Cisco Press

Wiki – Propagation Delay