Does gravity slow or speed light up? The speed of light in a vacuum is presumably the fastest speed possible.  

if gravity bends the course of light, does this imply that gravity the retards light so it is moving at a slower speed?  If it affects its course, why cannot gravity affect its speed -- or does it?

And if gravity does affect the speed of light, what does that say about our measurements of the distance to the farthest observable object?  Could we assume that all the gravity effects across 15 billion light-years even themselves out?  Or is the actual distance across the observable universe subject to unknowable variations due to gravity effects?
Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect.

Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the straight path in the absence of $M$. The light that reaches $O$ from $S$ is not emitted from $S$ in the "straight" (in the absence of $M$) direction to $O$, but slightly off that direction, such that the "bending" of its path by the gravity of $M$ "deflects" it onto $O$.

Of course, light is never bent, but always follows a straight path. What is bent is the space-time as compared to Euclidean space-time in the absence of distorting masses (see: geodesic). This distortion in the fabric of space time is called gravitational lensing.