minus two and a half times means...?

Here is a recent statement by a scientist.

"Cold water depresses the metabolic rate and the speed of fish," said [name deleted to spare embarrassment]. "For roughly every 10 degrees Celsius you drop the metabolic rates, speeds drop about two and a half times."

Think about it. if the speed drops by one time it is at zero. For it to drop more than one time, the fish is ...what .. going backwards?

Or I have seen it said that something is two times shorter than something else...huh?

Again, one time shorter would be at zero.

Thank you for reading.

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