Japanese Body Language – 頭の回転が遅い
Japanese Body Language –
頭の回転が遅い[あたまのかいてんがおそい, atama no kaiten ga osoi] be slow; be slow on the uptake; be dull or dimwitted
This expression is quite similar to atama ga warui except in my opinion, it's a bit more colorful.
kaiten is spinning or turning. And in the case of a dull person, it just doesn't spin fast enough.
The reverse is also said of someone who is pretty sharp – in that case,
頭の回転が速い[あたまのかいてんがはやい, atama no kaiten ga hayai] be quick; be quick on the update; be sharp
Which have you heard said about you?
What
