There are two yellow thieves in this game and both are charged on a race track. The first yellow thief is in a challenge area of the level Molten Crater and has about the same speed as a normal thief. The supercharge of the race track is not activated.
The second yellow thief is in a challenge area of Icy Peak. It's on a much trickier racing track and this one is a lot faster. However the supercharge is activated, so Spyro should be able to catch him.
Both times Spyro has to charge a much faster Red Thief afterwards.