>There is a race. Participants A and B.
>A finishes in 29 minutes
>B finishes in 35 minutes
>
>A is 6 minutes faster than B.
>B is 6 minutes slower than B.
>
>Is A 17% or 21% faster than B?
>Is B 17% or 21% slower than A?
>
>Title says it all, darn percentages :)

In any indication of percentage, the base value should be clear, otherwise there will be confusions. Sometimes it has to be explicitly stated, and of course, you'll get different results depending on the base value used.

A takes 17% less time than B (time for B is the base value), B takes 21% more time (time for A is the base value)

However, since you were asking about speed, the times have to be somehow converted to speeds. Either take an arbitrary distance to convert to speeds, or an unknown distance "x".

The speed of A is x km / 29 minutes, speed of B is x km / 35 minutes, so you can divide (x/29) / (x/35), making A go 35/29 as fast as B, or 21% faster. The base value in this case might be the speed of B, which is not directly stated in the problem.