Common game terminology taken from past games. I don't mean Sirlin games, I mean really old games like Chess that are many thousands of years old.
Speed most commonly will refer to how quickly you can implement some kind of win condition.
If you intend to kill the opponent by attacking with a tech 3 unit over and over, that would be a slow game plan. It just takes a lot of turns in order to be able to get out a tech 3 and then a lot more turns from there to win once you do.
If you had the intent to attack with a Void Star until the opponent died, you would be able to "start winning" (attack with the thing you intend to kill with) much more quickly. This would be "faster" than the above method.
In Chess terminology, speed in this context might be one master trying to get a quick win in the first 10 moves of the game vs another master trying to implement a plan where they win by building up small advantages over 30 turns.
Tempo most commonly will refer to how you feel when playing the game. Tempo is a measure of time. Not RL time, it's measured in something like "game turns" or similar.
Do you feel like you are winning or losing? If you feel like you are winning, you are probably ahead on tempo. If you feel like you are losing, you are probably behind on tempo. If you are spending your time just trying to stop your opponent from winning, you are behind in time. If you are spending your time trying to figure out how to most devastate your opponent with the things you have out, you are probably ahead on tempo. If it feels even but it's your turn, you are slightly ahead. The opposite if it feels even but it's their turn, then they are slightly ahead.
In chess terminology, Tempo at the beginning of the game would be slightly to white because it's an even board and it's white's turn. Black will typically have to do some sort of strong move that is both defensive and has multiple offensive threats in order to turn this around and make it so black is the attacker and white is the defender.