Translation:He beat the record by swimming even faster.
The S in plus is pronounced when the word has a positive meaning. It's silent when the meaning is negative.
But it's comparative modifying an adverb? https://frenchyourway.com.au/how-to-pronounce-plus-in-french
Am I to assume from this that this is how it's most commonly (or always) said in French? I mean "beat the record"? Though it is odd to an anglophone's ear, there's no point in trying to directly translate our way of saying something if it won't make sense! I do get why French learners find it frustrating when we have to learn something we'd never say that way, but we're not likely to forget something which sounds really weird to us! We just have to accept that this is how it's said, and for people learning English, we have ways of saying things that will be "wrong" to them too. That's languages for you (◠‿◕)