dim-witted; slow on the uptake
Idiomatic expression literally meaning 'poor circulation of blood (to the head)', used to describe someone who is slow to understand or not very sharp.
He's a dim-witted guy; no matter how many times you explain, he doesn't get it.
With someone slow on the uptake, you have to be blunt or they won't understand.
General adjective for 'dull' or 'slow'; can apply to senses or intellect, while 血の巡りの悪い is specifically about mental slowness.
Literally 'bad circulation of blood', an idiomatic phrase suggesting that poor blood flow to the brain causes slow thinking. The exact origin is uncertain, but it reflects a traditional folk belief.