'Slow' and 'slowly'

Slow 作為副詞，是所謂的「無詞尾變化副詞」(Flat Adverb) 。
其他例子有： come close (而不是 come closely) 、 hold tight (而不是 hold tightly) 等等。

以前，只有 slowly 才是副詞， slow 只是形容詞。
不過，後來漸漸地， slow 也被廣泛用作副詞。
根據這篇文章，大約在五百年前，牛津字典開始收錄 slow 作為副詞。

雖然 slowly 及 slow 都可作副詞，但使用上還是有點分別的。
《Merriam-Webster》 的 slow (adverb) 條下就有以下解釋。
有關動作的動詞幾乎一定配 slow 作副詞，而 slow 則會置在動詞後面，如： drive slow 。第一點的情況，用 slowly 亦都可以，只不過習慣上是用 slowslowly 則是用在動詞之前，如：slowly drive through the snowing 形式形容詞要配 slowly ，如： a slowly moving object總之，在任何情況，作為副詞的 slow 都可以用 slowly 取代，而且有些人認為嚴謹的寫作最好還是用 slowly ，不過那是非常非常嚴謹的說法。
事實上，符合以上第一點的話，用 slow 是慣用法，也不算錯。

American Heritage® Book of English Usage 就有以下說法：

Slow may sometimes be used instead of slowly when it comes after the verb: We drove the car slow. In formal writing, slowly is generally preferred. Slow is often used in speech and informal writing, especially when brevity and forcefulness are sought: Drive slow! Slow is also the established idiomatic form with certain senses of common verbs: The watch runs slow. Take it slow.

slow adjective
slowly adverb

go slow verb
go slowly adverb

go-slow=noun, meaning slowdown

go slowly=where slowly is an adv to decorate verb go.

They mean the same, just different ways to decribe the same thing. One can also say go slower.

Hope it helps.