People who speak slowly sound more intelligent than people who speak fast.
People who speak fast don't sound as smart as those who speak slowly.
ゆっくり話す人と早く話す人を比べているので、比較級を使ってあげます。
1) People who speak slowly sound more intelligent than people who speak fast.
「ゆっくり話す人は、早く話す人より賢く聞こえる。」
more intelligent than...で「…よりもっと賢い」
2) People who speak fast don't sound as smart as those who speak slowly.
「早く話す人は、ゆっくり話す人ほど賢く聞こえない。」
as smart as ... で「…くらい賢い」
「ゆっくり話す人」people who speak slowly
「早く話す人」people who speak fast
「賢い」intelligent または smart
ご参考になれば幸いです！
日本 
