The word “speed” came from the old German word sputen, to succeed or prosper. That’s what it originally meant in English, and that’s how Shakespeare’s using it here.

But to prosper in many activities simply requires that you perform them faster, which yielded our modern sense of “speed” — to move at a faster rate.

However, suppose you lived in the 19th century and went down to the harbor to see some friends off on a long sailing voyage. If you gave them the traditional blessing, “God speed”, you wouldn’t have been praying for God to make the ship sail fast, but simply to succeed in reaching its destination without sinking.