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 uptake; be sharp

Which have you heard said about you? Sharp as a tack or dumber than a box of rocks?