A lot of research on altering the speed of light in a localized space has been done by Evgeny Podkletnov and Giovanni Modanese.
The best sense I can make of it is: There are technologies (such as so-called gravity wave generators) which can clear out most of the virtual particles from the vacuum of space. Once you reduce the density of the virtual particles, light will now travel much faster than it does under normal virtual particle density conditions.
We perceive the speed of light as constant simply because in normal life, we are not in proximity to any technologies which alter the density of virtual particles.
fun reading:
http:// www.eurekaselect.com/100222/chapter/study-of-light-interaction-with-gravity-impulses-and-measurements-of-the-speed-of-gravity-impulse
https:// arxiv.org/pdf/1101.1063.pdf
http:// ethosworld.com/library/803-page-Collection-of-Papers-on-Anti-Gravity-Research.pdf