In general we expect this to work:

Quantity["Speed of light"]  Quantity[1, "SpeedOfLight"] 

And it does because it can make use of Wolfram|Alpha.

Oddly, though, when at work this will always timeout for me. The same happens with Entity when trying to format.

We can replicate it like so:

Block[{WolframAlphaClient`Private`$  AlphaQueryTimeConstraint = .1},  Quantity[1, "Speed of light"]  ] 

EntityValue::timeout: A network operation for EntityValue timed out. Please try again later.  Quantity::unkunit: Unable to interpret unit specification a. 

Quantity[1, "Speed of light"] 

Despite that, this does work:

WolframAlpha["speed of light", "Result"]  Quantity[1, "SpeedOfLight"] 

One thing I have noticed is that in my Terminal my hostname is reassigned to a UUID-like string rather than the standard human-readable one Macs provide.

Does anyone know why this might be happening?